diff --git a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
--- a/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
+++ b/clang-tools-extra/clangd/GlobalCompilationDatabase.cpp
@@ -395,20 +395,6 @@
   return None;
 }
 
-// For platforms where paths are case-insensitive (but case-preserving),
-// we need to do case-insensitive comparisons and use lowercase keys.
-// FIXME: Make Path a real class with desired semantics instead.
-//        This class is not the only place this problem exists.
-// FIXME: Mac filesystems default to case-insensitive, but may be sensitive.
-
-static std::string maybeCaseFoldPath(PathRef Path) {
-#if defined(_WIN32) || defined(__APPLE__)
-  return Path.lower();
-#else
-  return std::string(Path);
-#endif
-}
-
 std::vector<DirectoryBasedGlobalCompilationDatabase::DirectoryCache *>
 DirectoryBasedGlobalCompilationDatabase::getDirectoryCaches(
     llvm::ArrayRef<llvm::StringRef> Dirs) const {
diff --git a/clang-tools-extra/clangd/refactor/Rename.cpp b/clang-tools-extra/clangd/refactor/Rename.cpp
--- a/clang-tools-extra/clangd/refactor/Rename.cpp
+++ b/clang-tools-extra/clangd/refactor/Rename.cpp
@@ -68,7 +68,7 @@
     if (OtherFile)
       return;
     if (auto RefFilePath = filePath(R.Location, /*HintFilePath=*/MainFile)) {
-      if (*RefFilePath != MainFile)
+      if (!pathEqual(*RefFilePath, MainFile))
         OtherFile = *RefFilePath;
     }
   });
@@ -474,7 +474,7 @@
     if ((R.Kind & RefKind::Spelled) == RefKind::Unknown)
       return;
     if (auto RefFilePath = filePath(R.Location, /*HintFilePath=*/MainFile)) {
-      if (*RefFilePath != MainFile)
+      if (!pathEqual(*RefFilePath, MainFile))
         AffectedFiles[*RefFilePath].push_back(toRange(R.Location));
     }
   });
diff --git a/clang-tools-extra/clangd/support/CMakeLists.txt b/clang-tools-extra/clangd/support/CMakeLists.txt
--- a/clang-tools-extra/clangd/support/CMakeLists.txt
+++ b/clang-tools-extra/clangd/support/CMakeLists.txt
@@ -23,6 +23,7 @@
   Logger.cpp
   Markup.cpp
   MemoryTree.cpp
+  Path.cpp
   Shutdown.cpp
   Threading.cpp
   ThreadsafeFS.cpp
diff --git a/clang-tools-extra/clangd/support/Path.h b/clang-tools-extra/clangd/support/Path.h
--- a/clang-tools-extra/clangd/support/Path.h
+++ b/clang-tools-extra/clangd/support/Path.h
@@ -22,6 +22,12 @@
 /// signatures.
 using PathRef = llvm::StringRef;
 
+// For platforms where paths are case-insensitive (but case-preserving),
+// we need to do case-insensitive comparisons and use lowercase keys.
+// FIXME: Make Path a real class with desired semantics instead.
+std::string maybeCaseFoldPath(PathRef Path);
+bool pathEqual(PathRef, PathRef);
+
 } // namespace clangd
 } // namespace clang
 
diff --git a/clang-tools-extra/clangd/support/Path.cpp b/clang-tools-extra/clangd/support/Path.cpp
new file mode 100644
--- /dev/null
+++ b/clang-tools-extra/clangd/support/Path.cpp
@@ -0,0 +1,30 @@
+//===--- Path.cpp -------------------------------------------*- C++-*------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "support/Path.h"
+namespace clang {
+namespace clangd {
+
+std::string maybeCaseFoldPath(PathRef Path) {
+#if defined(_WIN32) || defined(__APPLE__)
+  return Path.lower();
+#else
+  return std::string(Path);
+#endif
+}
+
+bool pathEqual(PathRef A, PathRef B) {
+#if defined(_WIN32) || defined(__APPLE__)
+  return A.equals_lower(B);
+#else
+  return A == B;
+#endif
+}
+
+} // namespace clangd
+} // namespace clang
diff --git a/clang-tools-extra/clangd/unittests/RenameTests.cpp b/clang-tools-extra/clangd/unittests/RenameTests.cpp
--- a/clang-tools-extra/clangd/unittests/RenameTests.cpp
+++ b/clang-tools-extra/clangd/unittests/RenameTests.cpp
@@ -1067,6 +1067,52 @@
   }
 }
 
+MATCHER_P(newText, T, "") { return arg.newText == T; }
+
+TEST(RenameTest, IndexMergeMainFile) {
+  Annotations Code("int ^x();");
+  TestTU TU = TestTU::withCode(Code.code());
+  TU.Filename = "main.cc";
+  auto AST = TU.build();
+
+  auto Main = testPath("main.cc");
+
+  auto Rename = [&](const SymbolIndex *Idx) {
+    auto GetDirtyBuffer = [&](PathRef Path) -> llvm::Optional<std::string> {
+      return Code.code().str(); // Every file has the same content.
+    };
+    RenameOptions Opts;
+    Opts.AllowCrossFile = true;
+    RenameInputs Inputs{Code.point(), "xPrime", AST,           Main,
+                        Idx,          Opts,     GetDirtyBuffer};
+    auto Results = rename(Inputs);
+    EXPECT_TRUE(bool(Results)) << llvm::toString(Results.takeError());
+    return std::move(*Results);
+  };
+
+  // We do not expect to see duplicated edits from AST vs index.
+  auto Results = Rename(TU.index().get());
+  EXPECT_THAT(Results.GlobalChanges.keys(), ElementsAre(Main));
+  EXPECT_THAT(Results.GlobalChanges[Main].asTextEdits(),
+              ElementsAre(newText("xPrime")));
+
+  // Sanity check: we do expect to see index results!
+  TU.Filename = "other.cc";
+  Results = Rename(TU.index().get());
+  EXPECT_THAT(Results.GlobalChanges.keys(),
+              UnorderedElementsAre(Main, testPath("other.cc")));
+
+#if defined(_WIN32) || defined(__APPLE__)
+  // On case-insensitive systems, no duplicates if AST vs index case differs.
+  // https://github.com/clangd/clangd/issues/665
+  TU.Filename = "MAIN.CC";
+  Results = Rename(TU.index().get());
+  EXPECT_THAT(Results.GlobalChanges.keys(), ElementsAre(Main));
+  EXPECT_THAT(Results.GlobalChanges[Main].asTextEdits(),
+              ElementsAre(newText("xPrime")));
+#endif
+}
+
 TEST(RenameTest, MainFileReferencesOnly) {
   // filter out references not from main file.
   llvm::StringRef Test =