diff --git a/mlir/lib/Parser/Parser.cpp b/mlir/lib/Parser/Parser.cpp
--- a/mlir/lib/Parser/Parser.cpp
+++ b/mlir/lib/Parser/Parser.cpp
@@ -1676,6 +1676,7 @@
   if (parseToken(Token::l_brace, "expected '{' in attribute dictionary"))
     return failure();
 
+  llvm::SmallDenseSet<Identifier> seenKeys;
   auto parseElt = [&]() -> ParseResult {
     // The name of an attribute can either be a bare identifier, or a string.
     Optional<Identifier> nameId;
@@ -1686,6 +1687,8 @@
       nameId = builder.getIdentifier(getTokenSpelling());
     else
       return emitError("expected attribute name");
+    if (!seenKeys.insert(*nameId).second)
+      return emitError("duplicate key in dictionary attribute");
     consumeToken();
 
     // Try to parse the '=' for the attribute value.
diff --git a/mlir/test/IR/invalid.mlir b/mlir/test/IR/invalid.mlir
--- a/mlir/test/IR/invalid.mlir
+++ b/mlir/test/IR/invalid.mlir
@@ -1489,3 +1489,10 @@
   } : () -> ()
   return
 }
+
+// -----
+
+func @duplicate_dictionary_attr_key() {
+  // expected-error @+1 {{duplicate key in dictionary attribute}}
+  "foo.op"() {a, a} : () -> ()
+}