diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/PlatformMappingFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/PlatformMappingFunction.java
index 33694f4..5ae21de 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/PlatformMappingFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/PlatformMappingFunction.java
@@ -185,7 +185,12 @@
         platformsToFlags.put(platform, flags);
       }
 
-      return platformsToFlags.build();
+      try {
+        return platformsToFlags.build();
+      } catch (IllegalArgumentException e) {
+        throw throwParsingException(
+            e, "Got duplicate platform entries but each platform key must be unique");
+      }
     }
 
     private Label platform() throws PlatformMappingException {
@@ -202,9 +207,7 @@
         // mappings however only apply within a repository imported by the root repository.
         platform = Label.parseAbsolute(label, emptyRepositoryMapping);
       } catch (LabelSyntaxException e) {
-        throw new PlatformMappingException(
-            new PlatformMappingParsingException("Expected platform label but got " + label, e),
-            SkyFunctionException.Transience.PERSISTENT);
+        throw throwParsingException(e, "Expected platform label but got " + label);
       }
       return platform;
     }
@@ -234,7 +237,12 @@
         Label platform = platform();
         flagsToPlatforms.put(flags, platform);
       }
-      return flagsToPlatforms.build();
+      try {
+        return flagsToPlatforms.build();
+      } catch (IllegalArgumentException e) {
+        throw throwParsingException(
+            e, "Got duplicate flags entries but each flags key must be unique");
+      }
     }
 
     private String consume() {
@@ -251,6 +259,13 @@
       return line.get();
     }
 
+    private AssertionError throwParsingException(Exception e, String message)
+        throws PlatformMappingException {
+      throw new PlatformMappingException(
+          new PlatformMappingParsingException(message, e),
+          SkyFunctionException.Transience.PERSISTENT);
+    }
+
     private void throwParsingException(String message) throws PlatformMappingException {
       throw new PlatformMappingException(
           new PlatformMappingParsingException(message), SkyFunctionException.Transience.PERSISTENT);
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/PlatformMappingFunctionParserTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/PlatformMappingFunctionParserTest.java
index 6d7a9fa..f4796f9 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/PlatformMappingFunctionParserTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/PlatformMappingFunctionParserTest.java
@@ -274,17 +274,16 @@
 
     assertThat(exception).hasMessageThat().contains("platform:");
 
-    PlatformMappingFunction.PlatformMappingException exception2 =
-        assertThrows(
-            PlatformMappingFunction.PlatformMappingException.class,
-            () ->
-                parse(
-                    "platforms:",
-                    "  //platforms:one",
-                    "    --cpu=one",
-                    "flag:",
-                    "  --cpu=one",
-                    "    //platforms:one"));
+    assertThrows(
+        PlatformMappingFunction.PlatformMappingException.class,
+        () ->
+            parse(
+                "platforms:",
+                "  //platforms:one",
+                "    --cpu=one",
+                "flag:",
+                "  --cpu=one",
+                "    //platforms:one"));
 
     assertThat(exception).hasMessageThat().contains("platform");
   }
@@ -347,6 +346,51 @@
     assertThat(exception).hasMessageThat().contains("-cpu");
   }
 
+  @Test
+  public void testParsePlatformsDuplicatePlatform() throws Exception {
+    PlatformMappingFunction.PlatformMappingException exception =
+        assertThrows(
+            PlatformMappingFunction.PlatformMappingException.class,
+            () ->
+                parse(
+                    "platforms:", // Force line break
+                    "  //platforms:one", // Force line break
+                    "    --cpu=one", // Force line break
+                    "  //platforms:one", // Force line break
+                    "    --cpu=two"));
+
+    assertThat(exception).hasMessageThat().contains("duplicate");
+    assertThat(exception)
+        .hasCauseThat()
+        .hasCauseThat()
+        .hasMessageThat()
+        .contains("//platforms:one");
+  }
+
+  @Test
+  public void testParseFlagsDuplicateFlags() throws Exception {
+    PlatformMappingFunction.PlatformMappingException exception =
+        assertThrows(
+            PlatformMappingFunction.PlatformMappingException.class,
+            () ->
+                parse(
+                    "flags:", // Force line break
+                    "  --compilation_mode=dbg", // Force line break
+                    "  --cpu=one", // Force line break
+                    "    //platforms:one", // Force line break
+                    "  --compilation_mode=dbg", // Force line break
+                    "  --cpu=one", // Force line break
+                    "    //platforms:two"));
+
+    assertThat(exception).hasMessageThat().contains("duplicate");
+    assertThat(exception).hasCauseThat().hasCauseThat().hasMessageThat().contains("--cpu=one");
+    assertThat(exception)
+        .hasCauseThat()
+        .hasCauseThat()
+        .hasMessageThat()
+        .contains("--compilation_mode=dbg");
+  }
+
   private PlatformMappingFunction.Mappings parse(String... lines)
       throws PlatformMappingFunction.PlatformMappingException {
     return new PlatformMappingFunction.Parser(ImmutableList.copyOf(lines).iterator()).parse();