diff --git a/llvm/test/TableGen/RegClassByHwModeErrors.td b/llvm/test/TableGen/RegClassByHwModeErrors.td
index 0ee6370ccd0ce..c7731312e28a6 100644
--- a/llvm/test/TableGen/RegClassByHwModeErrors.td
+++ b/llvm/test/TableGen/RegClassByHwModeErrors.td
@@ -9,6 +9,8 @@
 // RUN:   %t/compress-regclass-by-hwmode-2.td -o /dev/null 2>&1 | FileCheck %t/compress-regclass-by-hwmode-2.td --implicit-check-not="error:"
 // RUN: not llvm-tblgen --gen-dag-isel  -I %p/../../include -I %t -I %S \
 // RUN:   %t/vt-by-hwmode-missing.td -o /dev/null 2>&1 | FileCheck %t/vt-by-hwmode-missing.td --implicit-check-not="error:"
+// RUN: not llvm-tblgen --gen-dag-isel -I %p/../../include -I %t -I %S \
+// RUN:   %t/multiple-entries-for-same-mode.td -o /dev/null 2>&1 | FileCheck %t/multiple-entries-for-same-mode.td --implicit-check-not="error:"
 
 //--- Common.td
 include "Common/RegClassByHwModeCommon.td"
@@ -119,3 +121,22 @@ def TEST : TestInstruction {
 
 def MyTargetISA : InstrInfo;
 def MyTarget : Target { let InstructionSet = MyTargetISA; }
+
+
+//--- multiple-entries-for-same-mode.td
+include "Common.td"
+/// We should get an error if the same mode is listed more than once
+defvar Ptr64Alias = Ptr64;
+def BadRegClass : RegClassByHwMode<[Ptr32, Ptr64, Ptr64Alias], [XRegs, YRegs, YRegs]>;
+// CHECK: [[#@LINE-1]]:5: error: duplicate RegisterClass entry for HwMode Ptr64: YRegs
+// Need at least one CompressPat use of the bad reg class to trigger the error:
+def USE_BAD_REG_CLASS : TestInstruction {
+  let OutOperandList = (outs BadRegClass:$dst);
+  let InOperandList = (ins BadRegClass:$src1, BadRegClass:$src2);
+  let AsmString = "bad $dst";
+  let Pattern = [(set BadRegClass:$dst, (add BadRegClass:$src1, BadRegClass:$src2))];
+}
+def MyTargetISA : InstrInfo;
+def MyTarget : Target {
+  let InstructionSet = MyTargetISA;
+}
diff --git a/llvm/utils/TableGen/Common/InfoByHwMode.cpp b/llvm/utils/TableGen/Common/InfoByHwMode.cpp
index a3f8909c36090..4ab27a610249d 100644
--- a/llvm/utils/TableGen/Common/InfoByHwMode.cpp
+++ b/llvm/utils/TableGen/Common/InfoByHwMode.cpp
@@ -18,6 +18,7 @@
 #include "llvm/ADT/Twine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/TableGen/Error.h"
 #include "llvm/TableGen/Record.h"
 #include <string>
 
@@ -32,10 +33,12 @@ std::string llvm::getModeName(unsigned Mode) {
 ValueTypeByHwMode::ValueTypeByHwMode(const Record *R, const CodeGenHwModes &CGH)
     : InfoByHwMode<llvm::MVT>(R) {
   const HwModeSelect &MS = CGH.getHwModeSelect(R);
-  for (const HwModeSelect::PairType &P : MS.Items) {
-    auto I = Map.try_emplace(P.first, MVT(llvm::getValueType(P.second)));
-    assert(I.second && "Duplicate entry?");
-    (void)I;
+  for (auto [ModeID, VT] : MS.Items) {
+    assert(VT && VT->isSubClassOf("ValueType"));
+    if (!Map.try_emplace(ModeID, MVT(llvm::getValueType(VT))).second)
+      PrintFatalError(R->getLoc(), "duplicate ValueType entry for HwMode " +
+                                       CGH.getModeName(ModeID, true) + ": " +
+                                       VT->getName());
   }
   if (R->isSubClassOf("PtrValueType"))
     PtrAddrSpace = R->getValueAsInt("AddrSpace");
@@ -143,10 +146,12 @@ RegSizeInfoByHwMode::RegSizeInfoByHwMode(const Record *R,
                                          const CodeGenHwModes &CGH)
     : InfoByHwMode<llvm::RegSizeInfo>(R) {
   const HwModeSelect &MS = CGH.getHwModeSelect(R);
-  for (const HwModeSelect::PairType &P : MS.Items) {
-    auto I = Map.try_emplace(P.first, RegSizeInfo(P.second));
-    assert(I.second && "Duplicate entry?");
-    (void)I;
+  for (auto [ModeID, RegInfo] : MS.Items) {
+    assert(RegInfo && RegInfo->isSubClassOf("RegInfo"));
+    if (!Map.try_emplace(ModeID, RegSizeInfo(RegInfo)).second)
+      PrintFatalError(R->getLoc(), "duplicate RegInfo entry for HwMode " +
+                                       CGH.getModeName(ModeID, true) + ": " +
+                                       RegInfo->getName());
   }
 }
 
@@ -198,7 +203,9 @@ RegClassByHwMode::RegClassByHwMode(const Record *R, const CodeGenHwModes &CGH,
            "Register class must subclass RegisterClass");
     const CodeGenRegisterClass *RegClass = RegBank.getRegClass(RegClassRec);
     if (!Map.try_emplace(ModeID, RegClass).second)
-      llvm_unreachable("duplicate entry");
+      PrintFatalError(R->getLoc(), "duplicate RegisterClass entry for HwMode " +
+                                       CGH.getModeName(ModeID, true) + ": " +
+                                       RegClass->getName());
   }
 }
 
@@ -211,10 +218,12 @@ SubRegRangeByHwMode::SubRegRangeByHwMode(const Record *R,
                                          const CodeGenHwModes &CGH)
     : InfoByHwMode<llvm::SubRegRange>(R) {
   const HwModeSelect &MS = CGH.getHwModeSelect(R);
-  for (const HwModeSelect::PairType &P : MS.Items) {
-    auto I = Map.try_emplace(P.first, SubRegRange(P.second));
-    assert(I.second && "Duplicate entry?");
-    (void)I;
+  for (auto [ModeID, Range] : MS.Items) {
+    assert(Range && Range->isSubClassOf("SubRegRange"));
+    if (!Map.try_emplace(ModeID, SubRegRange(Range)).second)
+      PrintFatalError(R->getLoc(), "duplicate SubRegRange entry for HwMode " +
+                                       CGH.getModeName(ModeID, true) + ": " +
+                                       Range->getName());
   }
 }
 
@@ -222,12 +231,14 @@ EncodingInfoByHwMode::EncodingInfoByHwMode(const Record *R,
                                            const CodeGenHwModes &CGH)
     : InfoByHwMode<const llvm::Record *>(R) {
   const HwModeSelect &MS = CGH.getHwModeSelect(R);
-  for (const HwModeSelect::PairType &P : MS.Items) {
-    assert(P.second && P.second->isSubClassOf("InstructionEncoding") &&
+  for (auto [ModeID, Encoding] : MS.Items) {
+    assert(Encoding && Encoding->isSubClassOf("InstructionEncoding") &&
            "Encoding must subclass InstructionEncoding");
-    auto I = Map.try_emplace(P.first, P.second);
-    assert(I.second && "Duplicate entry?");
-    (void)I;
+    if (!Map.try_emplace(ModeID, Encoding).second)
+      PrintFatalError(R->getLoc(),
+                      "duplicate InstructionEncoding entry for HwMode " +
+                          CGH.getModeName(ModeID, true) + ": " +
+                          Encoding->getName());
   }
 }