Index: llvm/trunk/lib/MC/MCParser/AsmParser.cpp
===================================================================
--- llvm/trunk/lib/MC/MCParser/AsmParser.cpp
+++ llvm/trunk/lib/MC/MCParser/AsmParser.cpp
@@ -3874,6 +3874,12 @@
     if (parseIdentifier(Parameter.Name))
       return TokError("expected identifier in '.macro' directive");
 
+    // Emit an error if two (or more) named parameters share the same name
+    for (const MCAsmMacroParameter& CurrParam : Parameters)
+      if (CurrParam.Name.equals(Parameter.Name))
+        return TokError("macro '" + Name + "' has multiple parameters"
+                        " named '" + Parameter.Name + "'");
+
     if (Lexer.is(AsmToken::Colon)) {
       Lex();  // consume ':'
 
Index: llvm/trunk/test/MC/AsmParser/macro-duplicate-params-names-err.s
===================================================================
--- llvm/trunk/test/MC/AsmParser/macro-duplicate-params-names-err.s
+++ llvm/trunk/test/MC/AsmParser/macro-duplicate-params-names-err.s
@@ -0,0 +1,7 @@
+// RUN: not llvm-mc %s 2> %t
+// RUN: FileCheck < %t %s
+
+.macro M a a
+.endm
+
+// CHECK: macro 'M' has multiple parameters named 'a'