diff --git a/clang-tools-extra/clangd/index/FileIndex.cpp b/clang-tools-extra/clangd/index/FileIndex.cpp
--- a/clang-tools-extra/clangd/index/FileIndex.cpp
+++ b/clang-tools-extra/clangd/index/FileIndex.cpp
@@ -236,7 +236,7 @@
       llvm::make_unique<RefSlab>(std::move(Contents.second)),
       /*CountReferences=*/true);
   MainFileIndex.reset(
-      MainFileSymbols.buildIndex(IndexType::Light, DuplicateHandling::PickOne));
+      MainFileSymbols.buildIndex(IndexType::Light, DuplicateHandling::Merge));
 }
 
 } // namespace clangd
diff --git a/clang-tools-extra/clangd/unittests/FileIndexTests.cpp b/clang-tools-extra/clangd/unittests/FileIndexTests.cpp
--- a/clang-tools-extra/clangd/unittests/FileIndexTests.cpp
+++ b/clang-tools-extra/clangd/unittests/FileIndexTests.cpp
@@ -46,6 +46,7 @@
 }
 MATCHER_P(QName, N, "") { return (arg.Scope + arg.Name).str() == N; }
 MATCHER_P(NumReferences, N, "") { return arg.References == N; }
+MATCHER_P(hasOrign, O, "") { return bool(arg.Origin & O); }
 
 namespace clang {
 namespace clangd {
@@ -367,6 +368,27 @@
               RefsAre({RefRange(Main.range())}));
 }
 
+TEST(FileIndexTest, MergeMainFileSymbols) {
+  const char* CommonHeader = "void foo();";
+  TestTU Header = TestTU::withCode(CommonHeader);
+  TestTU Cpp = TestTU::withCode("void foo() {}");
+  Cpp.Filename = "foo.cpp";
+  Cpp.HeaderFilename = "foo.h";
+  Cpp.HeaderCode = CommonHeader;
+
+  FileIndex Index;
+  auto HeaderAST = Header.build();
+  auto CppAST = Cpp.build();
+  Index.updateMain(testPath("foo.h"), HeaderAST);
+  Index.updateMain(testPath("foo.cpp"), CppAST);
+
+  auto Symbols = runFuzzyFind(Index, "");
+  // Check foo is merged, foo in Cpp wins (as we see the definition there).
+  EXPECT_THAT(Symbols, ElementsAre(AllOf(DeclURI("unittest:///foo.h"),
+                                         DefURI("unittest:///foo.cpp"),
+                                         hasOrign(SymbolOrigin::Merge))));
+}
+
 TEST(FileSymbolsTest, CountReferencesNoRefSlabs) {
   FileSymbols FS;
   FS.update("f1", numSlab(1, 3), nullptr, true);