diff --git a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
index d2d4b80..b1b13d0 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
@@ -909,16 +909,16 @@
                         .executionPlatform(getToolchainContext().executionPlatform().label())
                         .build());
     BuildOptions fromOptions = getConfiguration().getOptions();
-    List<BuildOptions> splitOptions = transition.split(fromOptions);
+    Map<String, BuildOptions> splitOptions = transition.split(fromOptions);
     List<ConfiguredTargetAndData> deps = getConfiguredTargetAndTargetDeps(attributeName);
 
-    if (SplitTransition.equals(fromOptions, splitOptions)) {
+    if (SplitTransition.equals(fromOptions, splitOptions.values())) {
       // The split transition is not active. Defer the decision on which CPU to use.
       return ImmutableMap.of(Optional.<String>absent(), deps);
     }
 
     Set<String> cpus = new HashSet<>();
-    for (BuildOptions options : splitOptions) {
+    for (BuildOptions options : splitOptions.values()) {
       // This method should only be called when the split config is enabled on the command line, in
       // which case this cpu can't be null.
       cpus.add(options.get(CoreOptions.class).cpu);
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/config/StarlarkDefinedConfigTransition.java b/src/main/java/com/google/devtools/build/lib/analysis/config/StarlarkDefinedConfigTransition.java
index e59a539..817e73e 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/config/StarlarkDefinedConfigTransition.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/config/StarlarkDefinedConfigTransition.java
@@ -101,7 +101,7 @@
    * @throws EvalException if there is an error evaluating the transition
    * @throws InterruptedException if evaluating the transition is interrupted
    */
-  public abstract ImmutableList<Map<String, Object>> evaluate(
+  public abstract ImmutableMap<String, Map<String, Object>> evaluate(
       Map<String, Object> previousSettings, StructImpl attributeMap)
       throws EvalException, InterruptedException;
 
@@ -134,9 +134,9 @@
     }
 
     @Override
-    public ImmutableList<Map<String, Object>> evaluate(
+    public ImmutableMap<String, Map<String, Object>> evaluate(
         Map<String, Object> previousSettings, StructImpl attributeMapper) {
-      return ImmutableList.of(changedSettings);
+      return ImmutableMap.of("analysis_test", changedSettings);
     }
 
     @Override
@@ -206,7 +206,7 @@
     // TODO(bazel-team): integrate dict-of-dicts return type with ctx.split_attr
     @Override
     @SuppressWarnings("rawtypes")
-    public ImmutableList<Map<String, Object>> evaluate(
+    public ImmutableMap<String, Map<String, Object>> evaluate(
         Map<String, Object> previousSettings, StructImpl attributeMapper)
         throws EvalException, InterruptedException {
       Object result;
@@ -222,7 +222,7 @@
         // settings. Return early for now since better error reporting will happen in
         // {@link FunctionTransitionUtil#validateFunctionOutputsMatchesDeclaredOutputs}
         if (((Dict) result).isEmpty()) {
-          return ImmutableList.of(ImmutableMap.of());
+          return ImmutableMap.of("error", ImmutableMap.of());
         }
         // TODO(bazel-team): integrate keys with ctx.split_attr. Currently ctx.split_attr always
         // keys on cpu value - we should be able to key on the keys returned here.
@@ -231,31 +231,38 @@
           Map<String, Dict> dictOfDict =
               ((Dict<?, ?>) result)
                   .getContents(String.class, Dict.class, "dictionary of options dictionaries");
-          ImmutableList.Builder<Map<String, Object>> builder = ImmutableList.builder();
+          ImmutableMap.Builder<String, Map<String, Object>> builder = ImmutableMap.builder();
           for (Map.Entry<String, Dict> entry : dictOfDict.entrySet()) { // rawtypes error
             Map<String, Object> dict =
                 ((Dict<?, ?>) entry.getValue())
                     .getContents(String.class, Object.class, "an option dictionary");
-            builder.add(dict);
+            builder.put(entry.getKey(), dict);
           }
           return builder.build();
         } catch (EvalException e) {
           // fall through
         }
         try {
-          return ImmutableList.of(
+          // Try if this is a non-split, i.e. 1:1, transition. In which case use the impl function
+          // name as the transition key though it would not be used anywhere.
+          return ImmutableMap.of(
+              impl.getName(),
               ((Dict<?, ?>) result)
                   .getContents(String.class, Object.class, "dictionary of options"));
         } catch (EvalException e) {
           throw new EvalException(impl.getLocation(), e.getMessage());
         }
       } else if (result instanceof Sequence) {
-        ImmutableList.Builder<Map<String, Object>> builder = ImmutableList.builder();
+        ImmutableMap.Builder<String, Map<String, Object>> builder = ImmutableMap.builder();
         try {
+          int i = 0;
           for (Dict<?, ?> toOptions :
               ((Sequence<?>) result)
                   .getContents(Dict.class, "dictionary of options dictionaries")) {
-            builder.add(toOptions.getContents(String.class, Object.class, "dictionary of options"));
+            // TODO(b/146347033): Document this behavior.
+            builder.put(
+                Integer.toString(i++),
+                toOptions.getContents(String.class, Object.class, "dictionary of options"));
           }
         } catch (EvalException e) {
           throw new EvalException(impl.getLocation(), e.getMessage());
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/config/transitions/SplitTransition.java b/src/main/java/com/google/devtools/build/lib/analysis/config/transitions/SplitTransition.java
index 525269c..625e625 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/config/transitions/SplitTransition.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/config/transitions/SplitTransition.java
@@ -15,10 +15,13 @@
 package com.google.devtools.build.lib.analysis.config.transitions;
 
 import com.google.common.base.Verify;
+import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
 import com.google.devtools.build.lib.analysis.config.BuildOptions;
 import com.google.devtools.build.lib.concurrent.ThreadSafety;
+import java.util.Collection;
 import java.util.List;
+import java.util.Map;
 
 /**
  * A configuration transition that maps a single input {@link BuildOptions} to possibly multiple
@@ -36,25 +39,26 @@
 @FunctionalInterface
 public interface SplitTransition extends ConfigurationTransition {
   /**
-   * Returns the list of {@code BuildOptions} after splitting, or the original options if this
-   * split is a noop.
+   * Returns the map of {@code BuildOptions} after splitting, or the original options if this split
+   * is a noop. The key values are used as dict keys in ctx.split_attr, so human-readable strings
+   * are recommended.
    *
    * <p>Returning an empty or null list triggers a {@link RuntimeException}.
    */
-  List<BuildOptions> split(BuildOptions buildOptions);
+  Map<String, BuildOptions> split(BuildOptions buildOptions);
 
   /**
    * Returns true iff {@code option} and {@splitOptions} are equal.
    *
    * <p>This can be used to determine if a split is a noop.
    */
-  static boolean equals(BuildOptions options, List<BuildOptions> splitOptions) {
+  static boolean equals(BuildOptions options, Collection<BuildOptions> splitOptions) {
     return splitOptions.size() == 1 && Iterables.getOnlyElement(splitOptions).equals(options);
   }
 
   @Override
   default List<BuildOptions> apply(BuildOptions buildOptions) {
-    List<BuildOptions> splitOptions = split(buildOptions);
+    List<BuildOptions> splitOptions = ImmutableList.copyOf(split(buildOptions).values());
     Verify.verifyNotNull(splitOptions, "Split transition output may not be null");
     Verify.verify(!splitOptions.isEmpty(), "Split transition output may not be empty");
     return splitOptions;
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionTransitionUtil.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionTransitionUtil.java
index 4441efd..58297d6 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionTransitionUtil.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/FunctionTransitionUtil.java
@@ -36,9 +36,9 @@
 import com.google.devtools.common.options.OptionsParser;
 import com.google.devtools.common.options.OptionsParsingException;
 import java.lang.reflect.Field;
+import java.util.Collection;
 import java.util.HashMap;
 import java.util.LinkedHashSet;
-import java.util.List;
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeMap;
@@ -63,7 +63,7 @@
    * @param attrObject the attributes of the rule to which this transition is attached
    * @return the post-transition build options.
    */
-  static List<BuildOptions> applyAndValidate(
+  static Map<String, BuildOptions> applyAndValidate(
       BuildOptions buildOptions,
       StarlarkDefinedConfigTransition starlarkTransition,
       StructImpl attrObject)
@@ -73,16 +73,16 @@
     Map<String, OptionInfo> optionInfoMap = buildOptionInfo(buildOptions);
     Dict<String, Object> settings = buildSettings(buildOptions, optionInfoMap, starlarkTransition);
 
-    ImmutableList.Builder<BuildOptions> splitBuildOptions = ImmutableList.builder();
+    ImmutableMap.Builder<String, BuildOptions> splitBuildOptions = ImmutableMap.builder();
 
-    ImmutableList<Map<String, Object>> transitions =
+    ImmutableMap<String, Map<String, Object>> transitions =
         starlarkTransition.evaluate(settings, attrObject);
-    validateFunctionOutputsMatchesDeclaredOutputs(transitions, starlarkTransition);
+    validateFunctionOutputsMatchesDeclaredOutputs(transitions.values(), starlarkTransition);
 
-    for (Map<String, Object> transition : transitions) {
+    for (Map.Entry<String, Map<String, Object>> entry : transitions.entrySet()) {
       BuildOptions transitionedOptions =
-          applyTransition(buildOptions, transition, optionInfoMap, starlarkTransition);
-      splitBuildOptions.add(transitionedOptions);
+          applyTransition(buildOptions, entry.getValue(), optionInfoMap, starlarkTransition);
+      splitBuildOptions.put(entry.getKey(), transitionedOptions);
     }
     return splitBuildOptions.build();
   }
@@ -93,7 +93,7 @@
    * StarlarkTransition#validate}
    */
   private static void validateFunctionOutputsMatchesDeclaredOutputs(
-      ImmutableList<Map<String, Object>> transitions,
+      Collection<Map<String, Object>> transitions,
       StarlarkDefinedConfigTransition starlarkTransition)
       throws EvalException {
     for (Map<String, Object> transition : transitions) {
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkAttributeTransitionProvider.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkAttributeTransitionProvider.java
index e0549aa..2767b44 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkAttributeTransitionProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkAttributeTransitionProvider.java
@@ -19,7 +19,7 @@
 
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Preconditions;
-import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.devtools.build.lib.analysis.config.BuildOptions;
 import com.google.devtools.build.lib.analysis.config.StarlarkDefinedConfigTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.SplitTransition;
@@ -36,7 +36,7 @@
 import com.google.devtools.build.lib.syntax.Printer;
 import com.google.devtools.build.lib.syntax.Starlark;
 import java.util.LinkedHashMap;
-import java.util.List;
+import java.util.Map;
 
 /**
  * This class implements {@link TransitionFactory} to provide a starlark-defined transition that
@@ -102,10 +102,9 @@
      *     error was encountered during transition application/validation.
      */
     @Override
-    public final List<BuildOptions> split(BuildOptions buildOptions) {
-      List<BuildOptions> toReturn;
+    public final Map<String, BuildOptions> split(BuildOptions buildOptions) {
       try {
-        toReturn = applyAndValidate(buildOptions, starlarkDefinedConfigTransition, attrObject);
+        return applyAndValidate(buildOptions, starlarkDefinedConfigTransition, attrObject);
       } catch (InterruptedException | EvalException e) {
         starlarkDefinedConfigTransition
             .getEventHandler()
@@ -113,9 +112,8 @@
                 Event.error(
                     starlarkDefinedConfigTransition.getLocationForErrorReporting(),
                     e.getMessage()));
-        return ImmutableList.of(buildOptions.clone());
+        return ImmutableMap.of("error", buildOptions.clone());
       }
-      return toReturn;
     }
   }
 }
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkRuleTransitionProvider.java b/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkRuleTransitionProvider.java
index 84bf516..9024366 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkRuleTransitionProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/skylark/StarlarkRuleTransitionProvider.java
@@ -32,7 +32,7 @@
 import com.google.devtools.build.lib.syntax.EvalException;
 import com.google.devtools.build.lib.syntax.Starlark;
 import java.util.LinkedHashMap;
-import java.util.List;
+import java.util.Map;
 import java.util.Objects;
 
 /**
@@ -106,7 +106,7 @@
     // attributes that are then configured by the outputs of these transitions.
     @Override
     public BuildOptions patch(BuildOptions buildOptions) {
-      List<BuildOptions> result;
+      Map<String, BuildOptions> result;
       try {
         result = applyAndValidate(buildOptions, starlarkDefinedConfigTransition, attrObject);
       } catch (EvalException | InterruptedException e) {
@@ -127,7 +127,7 @@
                     "Rule transition only allowed to return a single transitioned configuration."));
         return buildOptions.clone();
       }
-      return Iterables.getOnlyElement(result);
+      return Iterables.getOnlyElement(result.values());
     }
 
     @Override
diff --git a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidRuleClasses.java b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidRuleClasses.java
index 2ef4ecf..d022627 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/android/AndroidRuleClasses.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/android/AndroidRuleClasses.java
@@ -27,6 +27,7 @@
 import static com.google.devtools.build.lib.util.FileTypeSet.NO_FILE;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSortedSet;
 import com.google.common.collect.Lists;
 import com.google.devtools.build.lib.analysis.BaseRuleClasses;
@@ -66,6 +67,7 @@
 import com.google.devtools.build.lib.util.FileType;
 import com.google.devtools.build.lib.util.FileTypeSet;
 import java.util.List;
+import java.util.Map;
 
 /** Rule definitions for Android rules. */
 public final class AndroidRuleClasses {
@@ -235,7 +237,7 @@
     }
 
     @Override
-    public List<BuildOptions> split(BuildOptions buildOptions) {
+    public Map<String, BuildOptions> split(BuildOptions buildOptions) {
 
       AndroidConfiguration.Options androidOptions =
           buildOptions.get(AndroidConfiguration.Options.class);
@@ -247,19 +249,19 @@
         if (androidOptions.cpu.isEmpty()
             || androidCrosstoolTop == null
             || androidCrosstoolTop.equals(cppOptions.crosstoolTop)) {
-          return ImmutableList.of(buildOptions);
+          return ImmutableMap.of(buildOptions.get(CoreOptions.class).cpu, buildOptions);
 
         } else {
 
           BuildOptions splitOptions = buildOptions.clone();
           splitOptions.get(CoreOptions.class).cpu = androidOptions.cpu;
           setCommonAndroidOptions(androidOptions, splitOptions);
-          return ImmutableList.of(splitOptions);
+          return ImmutableMap.of(androidOptions.cpu, splitOptions);
         }
 
       } else {
 
-        ImmutableList.Builder<BuildOptions> result = ImmutableList.builder();
+        ImmutableMap.Builder<String, BuildOptions> result = ImmutableMap.builder();
         for (String cpu : ImmutableSortedSet.copyOf(androidOptions.fatApkCpus)) {
           BuildOptions splitOptions = buildOptions.clone();
           // Disable fat APKs for the child configurations.
@@ -270,7 +272,7 @@
           splitOptions.get(AndroidConfiguration.Options.class).cpu = cpu;
           splitOptions.get(CoreOptions.class).cpu = cpu;
           setCommonAndroidOptions(androidOptions, splitOptions);
-          result.add(splitOptions);
+          result.put(cpu, splitOptions);
         }
         return result.build();
       }
diff --git a/src/main/java/com/google/devtools/build/lib/rules/objc/MultiArchSplitTransitionProvider.java b/src/main/java/com/google/devtools/build/lib/rules/objc/MultiArchSplitTransitionProvider.java
index eda568f..fc0e372 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/objc/MultiArchSplitTransitionProvider.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/objc/MultiArchSplitTransitionProvider.java
@@ -20,7 +20,9 @@
 import com.google.common.base.Optional;
 import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.ImmutableSortedSet;
 import com.google.devtools.build.lib.analysis.RuleContext;
 import com.google.devtools.build.lib.analysis.config.BuildOptions;
 import com.google.devtools.build.lib.analysis.config.CoreOptions;
@@ -40,6 +42,7 @@
 import com.google.devtools.build.lib.syntax.Printer;
 import com.google.devtools.build.lib.syntax.StarlarkValue;
 import java.util.List;
+import java.util.Map;
 import java.util.stream.Collectors;
 
 /**
@@ -193,7 +196,7 @@
     }
 
     @Override
-    public final List<BuildOptions> split(BuildOptions buildOptions) {
+    public final Map<String, BuildOptions> split(BuildOptions buildOptions) {
       List<String> cpus;
       DottedVersion actualMinimumOsVersion;
       ConfigurationDistinguisher configurationDistinguisher;
@@ -259,7 +262,9 @@
           throw new IllegalArgumentException("Unsupported platform type " + platformType);
       }
 
-      ImmutableList.Builder<BuildOptions> splitBuildOptions = ImmutableList.builder();
+      // There may be some duplicate flag values.
+      cpus = ImmutableSortedSet.copyOf(cpus).asList();
+      ImmutableMap.Builder<String, BuildOptions> splitBuildOptions = ImmutableMap.builder();
       for (String cpu : cpus) {
         BuildOptions splitOptions = buildOptions.clone();
 
@@ -298,7 +303,7 @@
         }
 
         appleCommandLineOptions.configurationDistinguisher = configurationDistinguisher;
-        splitBuildOptions.add(splitOptions);
+        splitBuildOptions.put(cpu, splitOptions);
       }
       return splitBuildOptions.build();
     }
diff --git a/src/test/java/com/google/devtools/build/lib/analysis/CircularDependencyTest.java b/src/test/java/com/google/devtools/build/lib/analysis/CircularDependencyTest.java
index 3548847..89fca0e 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/CircularDependencyTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/CircularDependencyTest.java
@@ -22,7 +22,7 @@
 import static com.google.devtools.build.lib.packages.Type.STRING;
 import static com.google.devtools.build.lib.testutil.MoreAsserts.assertThrows;
 
-import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
 import com.google.devtools.build.lib.analysis.config.BuildOptions;
 import com.google.devtools.build.lib.analysis.config.CoreOptions;
@@ -273,7 +273,7 @@
                                 optionsFragment.commandLineBuildVariables.stream()
                                     .filter((pair) -> !pair.getKey().equals(define))
                                     .collect(toImmutableList());
-                            return ImmutableList.of(newOptions);
+                            return ImmutableMap.of("define_cleaner", newOptions);
                           };
                         }
 
diff --git a/src/test/java/com/google/devtools/build/lib/analysis/config/TransitionFactoriesTest.java b/src/test/java/com/google/devtools/build/lib/analysis/config/TransitionFactoriesTest.java
index 564dbcb..542504f 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/config/TransitionFactoriesTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/config/TransitionFactoriesTest.java
@@ -15,7 +15,7 @@
 
 import static com.google.common.truth.Truth.assertThat;
 
-import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.devtools.build.lib.analysis.config.transitions.NoTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.NullTransition;
 import com.google.devtools.build.lib.analysis.config.transitions.SplitTransition;
@@ -59,7 +59,7 @@
   public void splitTransition() {
     TransitionFactory<Object> factory =
         TransitionFactories.of(
-            (SplitTransition) buildOptions -> ImmutableList.of(buildOptions.clone()));
+            (SplitTransition) buildOptions -> ImmutableMap.of("test0", buildOptions.clone()));
     assertThat(factory).isNotNull();
     assertThat(factory.isHost()).isFalse();
     assertThat(factory.isSplit()).isTrue();
diff --git a/src/test/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionFactoryTest.java b/src/test/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionFactoryTest.java
index 492ae8a..c2ae62c 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionFactoryTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionFactoryTest.java
@@ -13,7 +13,7 @@
 // limitations under the License.
 package com.google.devtools.build.lib.analysis.config.transitions;
 
-import static com.google.common.collect.ImmutableList.toImmutableList;
+import static com.google.common.collect.ImmutableMap.toImmutableMap;
 import static com.google.common.truth.Truth.assertThat;
 
 import com.google.common.collect.ImmutableList;
@@ -23,6 +23,8 @@
 import com.google.devtools.build.lib.analysis.config.TransitionFactories;
 import com.google.devtools.build.lib.cmdline.Label;
 import java.util.List;
+import java.util.Map;
+import java.util.stream.IntStream;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -237,10 +239,13 @@
     }
 
     @Override
-    public List<BuildOptions> split(BuildOptions options) {
-      return flagValues.stream()
-          .map(value -> updateOptions(options, flagLabel, value))
-          .collect(toImmutableList());
+    public Map<String, BuildOptions> split(BuildOptions options) {
+      return IntStream.range(0, flagValues.size())
+          .boxed()
+          .collect(
+              toImmutableMap(
+                  i -> "stub_split" + i,
+                  i -> updateOptions(options, flagLabel, flagValues.get(i))));
     }
   }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionTest.java b/src/test/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionTest.java
index ed0e879..c621133 100644
--- a/src/test/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/analysis/config/transitions/ComposingTransitionTest.java
@@ -13,7 +13,7 @@
 // limitations under the License.
 package com.google.devtools.build.lib.analysis.config.transitions;
 
-import static com.google.common.collect.ImmutableList.toImmutableList;
+import static com.google.common.collect.ImmutableMap.toImmutableMap;
 import static com.google.common.truth.Truth.assertThat;
 
 import com.google.common.collect.ImmutableList;
@@ -22,6 +22,8 @@
 import com.google.devtools.build.lib.analysis.config.HostTransition;
 import com.google.devtools.build.lib.cmdline.Label;
 import java.util.List;
+import java.util.Map;
+import java.util.stream.IntStream;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -194,10 +196,13 @@
     }
 
     @Override
-    public List<BuildOptions> split(BuildOptions options) {
-      return flagValues.stream()
-          .map(value -> updateOptions(options, flagLabel, value))
-          .collect(toImmutableList());
+    public Map<String, BuildOptions> split(BuildOptions options) {
+      return IntStream.range(0, flagValues.size())
+          .boxed()
+          .collect(
+              toImmutableMap(
+                  i -> "stub_split" + i,
+                  i -> updateOptions(options, flagLabel, flagValues.get(i))));
     }
   }
 }
diff --git a/src/test/java/com/google/devtools/build/lib/packages/AttributeTest.java b/src/test/java/com/google/devtools/build/lib/packages/AttributeTest.java
index b805d85..6e903e0 100644
--- a/src/test/java/com/google/devtools/build/lib/packages/AttributeTest.java
+++ b/src/test/java/com/google/devtools/build/lib/packages/AttributeTest.java
@@ -23,7 +23,6 @@
 import static com.google.devtools.build.lib.testutil.MoreAsserts.assertThrows;
 
 import com.google.common.base.Predicates;
-import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.devtools.build.lib.analysis.config.BuildOptions;
 import com.google.devtools.build.lib.analysis.config.HostTransition;
@@ -40,6 +39,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -286,8 +286,8 @@
 
   private static class TestSplitTransition implements SplitTransition {
     @Override
-    public List<BuildOptions> split(BuildOptions buildOptions) {
-      return ImmutableList.of(buildOptions.clone(), buildOptions.clone());
+    public Map<String, BuildOptions> split(BuildOptions buildOptions) {
+      return ImmutableMap.of("test0", buildOptions.clone(), "test1", buildOptions.clone());
     }
   }
 
diff --git a/src/test/java/com/google/devtools/build/lib/query2/cquery/ConfiguredTargetQueryTest.java b/src/test/java/com/google/devtools/build/lib/query2/cquery/ConfiguredTargetQueryTest.java
index 7c196eb..ae83400 100644
--- a/src/test/java/com/google/devtools/build/lib/query2/cquery/ConfiguredTargetQueryTest.java
+++ b/src/test/java/com/google/devtools/build/lib/query2/cquery/ConfiguredTargetQueryTest.java
@@ -21,6 +21,7 @@
 import static com.google.devtools.build.lib.testutil.MoreAsserts.assertThrows;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Iterables;
 import com.google.devtools.build.lib.analysis.ConfiguredTarget;
 import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
@@ -45,7 +46,7 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.Iterator;
-import java.util.List;
+import java.util.Map;
 import java.util.Set;
 import java.util.stream.Collectors;
 import org.junit.Test;
@@ -271,12 +272,12 @@
     }
 
     @Override
-    public List<BuildOptions> split(BuildOptions options) {
+    public Map<String, BuildOptions> split(BuildOptions options) {
       BuildOptions result1 = options.clone();
       BuildOptions result2 = options.clone();
       result1.get(TestOptions.class).testArguments = Collections.singletonList(toOption1);
       result2.get(TestOptions.class).testArguments = Collections.singletonList(toOption2);
-      return ImmutableList.of(result1, result2);
+      return ImmutableMap.of("result1", result1, "result2", result2);
     }
   }
 
diff --git a/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcRuleTestCase.java b/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcRuleTestCase.java
index b6691b7..a32aeea 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcRuleTestCase.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcRuleTestCase.java
@@ -316,7 +316,7 @@
       throws InterruptedException, OptionsParsingException, InvalidConfigurationException {
     ImmutableList.Builder<BuildConfiguration> splitConfigs = ImmutableList.builder();
 
-    for (BuildOptions splitOptions : splitTransition.split(configuration.getOptions())) {
+    for (BuildOptions splitOptions : splitTransition.split(configuration.getOptions()).values()) {
       splitConfigs.add(getSkyframeExecutor().getConfigurationForTesting(
           reporter, configuration.fragmentClasses(), splitOptions));
     }
diff --git a/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcSkylarkTest.java b/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcSkylarkTest.java
index 3043343..ece89ea 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcSkylarkTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/objc/ObjcSkylarkTest.java
@@ -1413,6 +1413,26 @@
   }
 
   @Test
+  public void testMultiArchSplitTransitionWithDuplicateFlagValues() throws Exception {
+    scratch.file("examples/rule/BUILD");
+    writeObjcSplitTransitionTestFiles();
+
+    useConfiguration("--ios_multi_cpus=armv7,arm64,armv7");
+    ConfiguredTarget skylarkTarget = getConfiguredTarget("//examples/apple_skylark:my_target");
+    StructImpl myInfo = getMyInfoFromTarget(skylarkTarget);
+    ObjcProvider armv7Objc =
+        ((SkylarkInfo) myInfo.getValue("ios_armv7")).getValue("objc", ObjcProvider.class);
+    ObjcProvider arm64Objc =
+        ((SkylarkInfo) myInfo.getValue("ios_arm64")).getValue("objc", ObjcProvider.class);
+    assertThat(armv7Objc).isNotNull();
+    assertThat(arm64Objc).isNotNull();
+    assertThat(Iterables.getOnlyElement(armv7Objc.getObjcLibraries()).getExecPathString())
+        .contains("ios_armv7");
+    assertThat(Iterables.getOnlyElement(arm64Objc.getObjcLibraries()).getExecPathString())
+        .contains("ios_arm64");
+  }
+
+  @Test
   public void testNoSplitTransitionUsesCpuFlagValue() throws Exception {
     scratch.file("examples/rule/BUILD");
     writeObjcSplitTransitionTestFiles();
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/ConfigurationsForTargetsWithTrimmedConfigurationsTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/ConfigurationsForTargetsWithTrimmedConfigurationsTest.java
index 7e0fa75..94a99e3 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/ConfigurationsForTargetsWithTrimmedConfigurationsTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/ConfigurationsForTargetsWithTrimmedConfigurationsTest.java
@@ -52,6 +52,7 @@
 import com.google.devtools.build.lib.testutil.TestSpec;
 import com.google.devtools.build.lib.util.FileTypeSet;
 import java.util.List;
+import java.util.Map;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -68,27 +69,27 @@
 
   private static class NoopSplitTransition implements SplitTransition {
     @Override
-    public List<BuildOptions> split(BuildOptions buildOptions) {
-      return ImmutableList.of(buildOptions);
+    public Map<String, BuildOptions> split(BuildOptions buildOptions) {
+      return ImmutableMap.of("noop", buildOptions);
     }
   }
 
   private static class SetsHostCpuSplitTransition implements SplitTransition {
     @Override
-    public List<BuildOptions> split(BuildOptions buildOptions) {
+    public Map<String, BuildOptions> split(BuildOptions buildOptions) {
       BuildOptions result = buildOptions.clone();
       result.get(CoreOptions.class).hostCpu = "SET BY SPLIT";
-      return ImmutableList.of(result);
+      return ImmutableMap.of("hostCpu", result);
     }
   }
 
   private static class SetsCpuSplitTransition implements SplitTransition {
 
     @Override
-    public List<BuildOptions> split(BuildOptions buildOptions) {
+    public Map<String, BuildOptions> split(BuildOptions buildOptions) {
       BuildOptions result = buildOptions.clone();
       result.get(CoreOptions.class).cpu = "SET BY SPLIT";
-      return ImmutableList.of(result);
+      return ImmutableMap.of("cpu", result);
     }
   }
 
@@ -565,14 +566,14 @@
    */
   private static SplitTransition newSplitTransition(final String prefix) {
     return buildOptions -> {
-      ImmutableList.Builder<BuildOptions> result = ImmutableList.builder();
+      ImmutableMap.Builder<String, BuildOptions> result = ImmutableMap.builder();
       for (int index = 1; index <= 2; index++) {
         BuildOptions toOptions = buildOptions.clone();
         TestConfiguration.TestOptions baseOptions =
             toOptions.get(TestConfiguration.TestOptions.class);
         baseOptions.testFilter =
             (baseOptions.testFilter == null ? "" : baseOptions.testFilter) + prefix + index;
-        result.add(toOptions);
+        result.put(prefix + index, toOptions);
       }
       return result.build();
     };