diff --git a/clang/include/clang/AST/ASTContext.h b/clang/include/clang/AST/ASTContext.h
--- a/clang/include/clang/AST/ASTContext.h
+++ b/clang/include/clang/AST/ASTContext.h
@@ -53,6 +53,7 @@
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/PointerUnion.h"
+#include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/ADT/StringRef.h"
@@ -465,7 +466,7 @@
   /// a non-constant initializer), or an ImportDecl (which recursively triggers
   /// initialization of another module).
   struct PerModuleInitializers {
-    llvm::SmallVector<Decl*, 4> Initializers;
+    llvm::SmallSetVector<Decl *, 4> Initializers;
     llvm::SmallVector<uint32_t, 4> LazyInitializers;
 
     void resolve(ASTContext &Ctx);
@@ -1078,6 +1079,9 @@
   /// Get the initializations to perform when importing a module, if any.
   ArrayRef<Decl*> getModuleInitializers(Module *M);
 
+  /// Does the module initializer array contain this decl.
+  bool moduleInitializerContains(Module *M, const Decl *D);
+
   /// Set the (C++20) module we are building.
   void setModuleForCodeGen(Module *M) { TopLevelModule = M; }
 
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -1148,7 +1148,7 @@
   LazyInitializers.clear();
 
   for (auto ID : LazyInits)
-    Initializers.push_back(Source->GetExternalDecl(ID));
+    Initializers.insert(Source->GetExternalDecl(ID));
 
   assert(LazyInitializers.empty() &&
          "GetExternalDecl for lazy module initializer added more inits");
@@ -1177,7 +1177,7 @@
   auto *&Inits = ModuleInitializers[M];
   if (!Inits)
     Inits = new (*this) PerModuleInitializers;
-  Inits->Initializers.push_back(D);
+  Inits->Initializers.insert(D);
 }
 
 void ASTContext::addLazyModuleInitializers(Module *M, ArrayRef<uint32_t> IDs) {
@@ -1195,7 +1195,17 @@
 
   auto *Inits = It->second;
   Inits->resolve(*this);
-  return Inits->Initializers;
+  return Inits->Initializers.getArrayRef();
+}
+
+bool ASTContext::moduleInitializerContains(Module *M, const Decl *D) {
+  auto It = ModuleInitializers.find(M);
+  if (It == ModuleInitializers.end())
+    return false;
+
+  auto *Inits = It->second;
+  Inits->resolve(*this);
+  return Inits->Initializers.contains(const_cast<Decl *>(D));
 }
 
 ExternCContextDecl *ASTContext::getExternCContextDecl() const {