diff --git a/clang/lib/Lex/HeaderSearch.cpp b/clang/lib/Lex/HeaderSearch.cpp
--- a/clang/lib/Lex/HeaderSearch.cpp
+++ b/clang/lib/Lex/HeaderSearch.cpp
@@ -1167,12 +1167,12 @@
   HeaderFileInfo *HFI = &FileInfo[FE->getUID()];
   // FIXME: Use a generation count to check whether this is really up to date.
   if (ExternalSource && !HFI->Resolved) {
-    HFI->Resolved = true;
     auto ExternalHFI = ExternalSource->GetHeaderFileInfo(FE);
-
-    HFI = &FileInfo[FE->getUID()];
-    if (ExternalHFI.External)
-      mergeHeaderFileInfo(*HFI, ExternalHFI);
+    if (ExternalHFI.IsValid) {
+      HFI->Resolved = true;
+      if (ExternalHFI.External)
+        mergeHeaderFileInfo(*HFI, ExternalHFI);
+    }
   }
 
   HFI->IsValid = true;
@@ -1199,12 +1199,12 @@
     if (!WantExternal && (!HFI->IsValid || HFI->External))
       return nullptr;
     if (!HFI->Resolved) {
-      HFI->Resolved = true;
       auto ExternalHFI = ExternalSource->GetHeaderFileInfo(FE);
-
-      HFI = &FileInfo[FE->getUID()];
-      if (ExternalHFI.External)
-        mergeHeaderFileInfo(*HFI, ExternalHFI);
+      if (ExternalHFI.IsValid) {
+        HFI->Resolved = true;
+        if (ExternalHFI.External)
+          mergeHeaderFileInfo(*HFI, ExternalHFI);
+      }
     }
   } else if (FE->getUID() >= FileInfo.size()) {
     return nullptr;
diff --git a/clang/test/Modules/Inputs/import-once/ImportOnce.framework/Headers/ImportOnce.h b/clang/test/Modules/Inputs/import-once/ImportOnce.framework/Headers/ImportOnce.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/import-once/ImportOnce.framework/Headers/ImportOnce.h
@@ -0,0 +1,5 @@
+// No header guards on purpose.
+
+enum SomeSimpleEnum {
+    SomeSimpleEnumCase,
+};
diff --git a/clang/test/Modules/Inputs/import-once/ImportOnce.framework/Modules/module.modulemap b/clang/test/Modules/Inputs/import-once/ImportOnce.framework/Modules/module.modulemap
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/import-once/ImportOnce.framework/Modules/module.modulemap
@@ -0,0 +1,4 @@
+framework module ImportOnce {
+  umbrella header "ImportOnce.h"
+  export *
+}
diff --git a/clang/test/Modules/Inputs/import-once/IndirectImporter.framework/Headers/IndirectImporter.h b/clang/test/Modules/Inputs/import-once/IndirectImporter.framework/Headers/IndirectImporter.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/import-once/IndirectImporter.framework/Headers/IndirectImporter.h
@@ -0,0 +1,2 @@
+#import <Unrelated/Unrelated.h>
+#import <ImportOnce/ImportOnce.h>
diff --git a/clang/test/Modules/Inputs/import-once/IndirectImporter.framework/Modules/module.modulemap b/clang/test/Modules/Inputs/import-once/IndirectImporter.framework/Modules/module.modulemap
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/import-once/IndirectImporter.framework/Modules/module.modulemap
@@ -0,0 +1,4 @@
+framework module IndirectImporter {
+    umbrella header "IndirectImporter.h"
+    export *
+}
diff --git a/clang/test/Modules/Inputs/import-once/Unrelated.framework/Headers/Unrelated.h b/clang/test/Modules/Inputs/import-once/Unrelated.framework/Headers/Unrelated.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/import-once/Unrelated.framework/Headers/Unrelated.h
@@ -0,0 +1 @@
+void foo(void);
diff --git a/clang/test/Modules/Inputs/import-once/Unrelated.framework/Modules/module.modulemap b/clang/test/Modules/Inputs/import-once/Unrelated.framework/Modules/module.modulemap
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/import-once/Unrelated.framework/Modules/module.modulemap
@@ -0,0 +1,4 @@
+framework module Unrelated {
+    umbrella header "Unrelated.h"
+    export *
+}
diff --git a/clang/test/Modules/import-once.m b/clang/test/Modules/import-once.m
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/import-once.m
@@ -0,0 +1,15 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules -fmodule-name=ImportOnce -fimplicit-module-maps -fmodules-cache-path=%t -F %S/Inputs/import-once %s
+
+// Test #import-ed headers are processed only once, even without header guards.
+// Dependency graph is
+//
+//     Unrelated       ImportOnce
+//           ^          ^    ^
+//            \        /     |
+//       IndirectImporter    |
+//                     ^     |
+//                      \    |
+//                   import-once.m
+#import <IndirectImporter/IndirectImporter.h>
+#import <ImportOnce/ImportOnce.h>