diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -8181,8 +8181,18 @@
 /// Add the given set of methods to the method list.
 static void addMethodsToPool(Sema &S, ArrayRef<ObjCMethodDecl *> Methods,
                              ObjCMethodList &List) {
-  for (unsigned I = 0, N = Methods.size(); I != N; ++I) {
-    S.addMethodToGlobalList(&List, Methods[I]);
+  // Methods from visited modules can contain a lot of duplicates
+  // when redeclaring methods from system frameworks, for example
+  // when marking -(instancetype)init as NS_UNAVAILABLE.
+  llvm::DenseSet<ObjCMethodDecl *> seen;
+  for (auto *L = &List; L; L = L->getNext()) {
+    seen.insert(L->getMethod());
+  }
+
+  for (auto *M : Methods) {
+    if (seen.insert(M).second) {
+      S.addMethodToGlobalList(&List, M);
+    }
   }
 }