diff --git a/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp b/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
index 70b61c3ffae..1960c993793 100644
--- a/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
+++ b/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp
@@ -4132,7 +4132,7 @@ bool AArch64AsmParser::parseDirectiveReq(StringRef Name, SMLoc L) {
   Parser.Lex(); // Consume the EndOfStatement
 
   auto pair = std::make_pair(IsVector, RegNum);
-  if (!RegisterReqs.insert(std::make_pair(Name, pair)).second)
+  if (RegisterReqs.insert(std::make_pair(Name, pair)).first->second != pair)
     Warning(L, "ignoring redefinition of register alias '" + Name + "'");
 
   return true;
diff --git a/lib/Target/ARM/AsmParser/ARMAsmParser.cpp b/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
index c35b0ef311f..3b7a67d0d1f 100644
--- a/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
+++ b/lib/Target/ARM/AsmParser/ARMAsmParser.cpp
@@ -9010,7 +9010,7 @@ bool ARMAsmParser::parseDirectiveReq(StringRef Name, SMLoc L) {
 
   Parser.Lex(); // Consume the EndOfStatement
 
-  if (!RegisterReqs.insert(std::make_pair(Name, Reg)).second) {
+  if (RegisterReqs.insert(std::make_pair(Name, Reg)).first->second != Reg) {
     Error(SRegLoc, "redefinition of '" + Name + "' does not match original.");
     return false;
   }
diff --git a/test/MC/AArch64/dot-req.s b/test/MC/AArch64/dot-req.s
index 947f945bded..a557f0c6758 100644
--- a/test/MC/AArch64/dot-req.s
+++ b/test/MC/AArch64/dot-req.s
@@ -1,7 +1,9 @@
-// RUN: llvm-mc -triple=aarch64-none-linux-gnu -show-encoding < %s | FileCheck %s
+// RUN: llvm-mc -triple=aarch64-none-linux-gnu -show-encoding < %s 2>&1 | FileCheck %s
 
 bar:
         fred .req x5
+// CHECK-NOT: ignoring redefinition of register alias 'fred'
+        fred .req x5
         mov fred, x11
         .unreq fred
         fred .req w6
diff --git a/test/MC/ARM/dot-req.s b/test/MC/ARM/dot-req.s
index 3b4cf5c80c0..848c1241aff 100644
--- a/test/MC/ARM/dot-req.s
+++ b/test/MC/ARM/dot-req.s
@@ -1,6 +1,9 @@
 @ RUN: llvm-mc -triple=armv7-apple-darwin -show-encoding < %s | FileCheck %s
         .syntax unified
 bar:
+@ The line is duplicated on purpose, it is legal to redefine a req with
+@ the same value.
+fred .req r5
 fred .req r5
         mov r11, fred
 .unreq fred