Index: clangd/CodeComplete.cpp
===================================================================
--- clangd/CodeComplete.cpp
+++ clangd/CodeComplete.cpp
@@ -450,8 +450,14 @@
   void ProcessCodeCompleteResults(class Sema &S, CodeCompletionContext Context,
                                   CodeCompletionResult *InResults,
                                   unsigned NumResults) override final {
+    if (CCSema) {
+      log(llvm::formatv("ProcessCodeCompleteResults called multiple times."
+                        "Previous ContextKind: '{0}'. New ContextKind: '{1}'",
+                        getCompletionKindString(this->CCContext.getKind()),
+                        getCompletionKindString(Context.getKind())));
+      return;
+    }
     // Record the completion context.
-    assert(!CCSema && "ProcessCodeCompleteResults called multiple times!");
     CCSema = &S;
     CCContext = Context;
 
Index: unittests/clangd/CodeCompleteTests.cpp
===================================================================
--- unittests/clangd/CodeCompleteTests.cpp
+++ unittests/clangd/CodeCompleteTests.cpp
@@ -608,6 +608,42 @@
   EXPECT_THAT(Results.items, Not(Contains(Labeled("clang::"))));
 }
 
+TEST(CompletionTest, BacktrackCrashes) {
+  // Sema calls code completion callbacks twice in these cases.
+  auto Results = completions(R"cpp(
+      namespace ns {
+      struct FooBarBaz {};
+      } // namespace ns
+
+     int foo(ns::FooBar^
+  )cpp");
+
+  EXPECT_THAT(Results.items, ElementsAre(Labeled("FooBarBaz")));
+
+  Results = completions(R"cpp(
+    struct FooBarBaz {};
+    void test() {
+      if (FooBarBaz * x^) {}
+    }
+)cpp");
+}
+
+TEST(CompletionTest, CompleteInExcludedPPBranch) {
+  auto Results = completions(R"cpp(
+    int bar(int param_in_bar) {
+    }
+
+    int foo(int param_in_foo) {
+#if 0
+  par^
+#endif
+    }
+)cpp");
+
+  EXPECT_THAT(Results.items, Contains(Labeled("param_in_foo")));
+  EXPECT_THAT(Results.items, Not(Contains(Labeled("param_in_bar"))));
+}
+
 SignatureHelp signatures(StringRef Text) {
   MockFSProvider FS;
   MockCompilationDatabase CDB;