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/test/tools/yaml2obj/map_duplicate_key.test b/llvm/test/tools/yaml2obj/map_duplicate_key.test
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/yaml2obj/map_duplicate_key.test
@@ -0,0 +1,11 @@
+# RUN: not yaml2obj %s 2>&1 | FileCheck %s
+
+--- !ELF
+FileHeader:
+  Class:      ELFCLASS64
+  Data:       ELFDATA2LSB
+  Type:       ET_REL
+  Type:       ET_REL
+  ABIVersion: 0x05
+
+# CHECK: error: duplicated mapping key 'Type
diff --git a/llvm/test/tools/yaml2obj/map_duplicate_key_2.test b/llvm/test/tools/yaml2obj/map_duplicate_key_2.test
new file mode 100644
--- /dev/null
+++ b/llvm/test/tools/yaml2obj/map_duplicate_key_2.test
@@ -0,0 +1,15 @@
+# RUN: not yaml2obj %s 2>&1 | FileCheck %s
+
+--- !ELF
+FileHeader:
+  Class:      ELFCLASS64
+  Data:       ELFDATA2LSB
+  Type:       ET_REL
+  ABIVersion: 0x05
+FileHeader:
+  Class:      ELFCLASS64
+  Data:       ELFDATA2LSB
+  Type:       ET_REL
+  ABIVersion: 0x05
+
+# CHECK: error: duplicated mapping key 'FileHeader