diff --git a/clang-tools-extra/clangd/refactor/tweaks/PopulateSwitch.cpp b/clang-tools-extra/clangd/refactor/tweaks/PopulateSwitch.cpp
--- a/clang-tools-extra/clangd/refactor/tweaks/PopulateSwitch.cpp
+++ b/clang-tools-extra/clangd/refactor/tweaks/PopulateSwitch.cpp
@@ -40,6 +40,8 @@
 #include "clang/Basic/SourceLocation.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Tooling/Core/Replacement.h"
+#include "llvm/ADT/MapVector.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallSet.h"
 #include <cassert>
 #include <string>
@@ -112,15 +114,25 @@
   if (!EnumD)
     return false;
 
-  // We trigger if there are fewer cases than enum values (and no case covers
-  // multiple values). This guarantees we'll have at least one case to insert.
-  // We don't yet determine what the cases are, as that means evaluating
-  // expressions.
-  auto I = EnumD->enumerator_begin();
-  auto E = EnumD->enumerator_end();
+  // Special case of the empty enum
+  if (EnumD->enumerator_begin() == EnumD->enumerator_end())
+    return false;
+
+  ASTContext &Ctx = Sel.AST->getASTContext();
+
+  unsigned EnumIntWidth = Ctx.getIntWidth(QualType(EnumT, 0));
+  bool EnumIsSigned = EnumT->isSignedIntegerOrEnumerationType();
+
+  llvm::SmallMapVector<llvm::APSInt, bool, 32> EnumConstants;
+  for (auto *EnumConstant : EnumD->enumerators()) {
+    llvm::APSInt Val = EnumConstant->getInitVal();
+    Val = Val.extOrTrunc(EnumIntWidth);
+    Val.setIsSigned(EnumIsSigned);
+    EnumConstants.insert(std::make_pair(Val, false));
+  }
 
-  for (const SwitchCase *CaseList = Switch->getSwitchCaseList();
-       CaseList && I != E; CaseList = CaseList->getNextSwitchCase(), I++) {
+  for (const SwitchCase *CaseList = Switch->getSwitchCaseList(); CaseList;
+       CaseList = CaseList->getNextSwitchCase()) {
     // Default likely intends to cover cases we'd insert.
     if (isa<DefaultStmt>(CaseList))
       return false;
@@ -135,10 +147,20 @@
     const ConstantExpr *CE = dyn_cast<ConstantExpr>(CS->getLHS());
     if (!CE || CE->isValueDependent())
       return false;
+
+    if (CE->getResultStorageKind() != ConstantExpr::RSK_Int64)
+      return false;
+    llvm::APSInt Val = CE->getResultAsAPSInt();
+    Val = Val.extOrTrunc(EnumIntWidth);
+    Val.setIsSigned(EnumIsSigned);
+    auto *Iter = EnumConstants.find(Val);
+    if (Iter == EnumConstants.end())
+      return false;
+    Iter->second = true;
+    continue;
   }
 
-  // Only suggest tweak if we have more enumerators than cases.
-  return I != E;
+  return !llvm::all_of(EnumConstants, [](auto &Pair) { return Pair.second; });
 }
 
 Expected<Tweak::Effect> PopulateSwitch::apply(const Selection &Sel) {
@@ -166,7 +188,10 @@
 
   std::string Text;
   for (EnumConstantDecl *Enumerator : EnumD->enumerators()) {
-    if (ExistingEnumerators.contains(Enumerator->getInitVal()))
+    // Try to insert this Enumerator into the set. If this fails, the value was
+    // either already there to begin with or we have already added it using a
+    // different name.
+    if (!ExistingEnumerators.insert(Enumerator->getInitVal()).second)
       continue;
 
     Text += "case ";
diff --git a/clang-tools-extra/clangd/unittests/TweakTests.cpp b/clang-tools-extra/clangd/unittests/TweakTests.cpp
--- a/clang-tools-extra/clangd/unittests/TweakTests.cpp
+++ b/clang-tools-extra/clangd/unittests/TweakTests.cpp
@@ -2980,6 +2980,18 @@
             void function() { switch (ns::A) {case ns::A:break;} }
           )"",
       },
+      {
+          // Duplicated constant names
+          Function,
+          R""(enum Enum {A,B,b=B}; ^switch (A) {})"",
+          R""(enum Enum {A,B,b=B}; switch (A) {case A:case B:break;})"",
+      },
+      {
+          // Duplicated constant names all in switch
+          Function,
+          R""(enum Enum {A,B,b=B}; ^switch (A) {case A:case B:break;})"",
+          "unavailable",
+      },
   };
 
   for (const auto &Case : Cases) {
diff --git a/llvm/include/llvm/ADT/DenseMapInfo.h b/llvm/include/llvm/ADT/DenseMapInfo.h
--- a/llvm/include/llvm/ADT/DenseMapInfo.h
+++ b/llvm/include/llvm/ADT/DenseMapInfo.h
@@ -14,6 +14,7 @@
 #define LLVM_ADT_DENSEMAPINFO_H
 
 #include "llvm/ADT/APInt.h"
+#include "llvm/ADT/APSInt.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/StringRef.h"
@@ -371,6 +372,26 @@
   }
 };
 
+/// Provide DenseMapInfo for APSInt, using the DenseMapInfo for APInt.
+template <> struct DenseMapInfo<APSInt> {
+  static inline APSInt getEmptyKey() {
+    return APSInt(DenseMapInfo<APInt>::getEmptyKey());
+  }
+
+  static inline APSInt getTombstoneKey() {
+    return APSInt(DenseMapInfo<APInt>::getTombstoneKey());
+  }
+
+  static unsigned getHashValue(const APSInt &Key) {
+    return static_cast<unsigned>(hash_value(Key));
+  }
+
+  static bool isEqual(const APSInt &LHS, const APSInt &RHS) {
+    return LHS.getBitWidth() == RHS.getBitWidth() &&
+           LHS.isUnsigned() == RHS.isUnsigned() && LHS == RHS;
+  }
+};
+
 } // end namespace llvm
 
 #endif // LLVM_ADT_DENSEMAPINFO_H