diff --git a/llvm/lib/Support/YAMLTraits.cpp b/llvm/lib/Support/YAMLTraits.cpp
--- a/llvm/lib/Support/YAMLTraits.cpp
+++ b/llvm/lib/Support/YAMLTraits.cpp
@@ -435,6 +435,11 @@
         // Copy string to permanent storage
         KeyStr = StringStorage.str().copy(StringAllocator);
       }
+      if (mapHNode->Mapping.count(KeyStr))
+        // From YAML spec: "The content of a mapping node is an unordered set of
+        // key/value node pairs, with the restriction that each of the keys is
+        // unique."
+        setError(KeyNode, Twine("duplicated mapping key '") + KeyStr + "'");
       auto ValueHNode = createHNodes(Value);
       if (EC)
         break;
diff --git a/llvm/unittests/Support/YAMLIOTest.cpp b/llvm/unittests/Support/YAMLIOTest.cpp
--- a/llvm/unittests/Support/YAMLIOTest.cpp
+++ b/llvm/unittests/Support/YAMLIOTest.cpp
@@ -105,6 +105,16 @@
   EXPECT_TRUE(!!yin.error());
 }
 
+TEST(YAMLIO, TestMapDuplicatedKeysRead) {
+  auto testDiagnostic = [](const llvm::SMDiagnostic &Error, void *) {
+    EXPECT_EQ(Error.getMessage(), "duplicated mapping key 'foo'");
+  };
+  FooBar doc;
+  Input yin("{foo: 3, bar: 5, foo: 4}", nullptr, testDiagnostic);
+  yin >> doc;
+  EXPECT_TRUE(!!yin.error());
+}
+
 //
 // Test the reading of a yaml sequence of mappings
 //