diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index d4a3513..b6115ec 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -915,9 +915,9 @@
     // Search for a module with the given name.
     Module = PP->getHeaderSearchInfo().lookupModule(ModuleName);
     std::string ModuleFileName;
-    if (Module)
+    if (Module) {
       ModuleFileName = PP->getHeaderSearchInfo().getModuleFileName(Module);
-    else
+    } else
       ModuleFileName = PP->getHeaderSearchInfo().getModuleFileName(ModuleName);
 
     if (ModuleFileName.empty()) {
@@ -987,6 +987,20 @@
       return ModuleLoadResult();
     }
 
+    // If there is already a module file associated with this module, make sure
+    // it is the same as the module file we're looking for. Otherwise, we
+    // have two module files for the same module.
+    if (const FileEntry *CurModuleFile = Module? Module->getASTFile() : 0) {
+      if (CurModuleFile != ModuleFile) {
+        getDiagnostics().Report(ModuleNameLoc, diag::err_module_file_conflict)
+          << ModuleName
+          << CurModuleFile->getName()
+          << ModuleFile->getName();
+        ModuleBuildFailed = true;
+        return ModuleLoadResult();
+      }
+    }
+
     // If we don't already have an ASTReader, create one now.
     if (!ModuleManager) {
       if (!hasASTContext())
@@ -1085,8 +1099,9 @@
                  .findModule((Path[0].first->getName()));
     }
 
-    if (Module)
+    if (Module) {
       Module->setASTFile(ModuleFile);
+    }
     
     // Cache the result of this top-level module lookup for later.
     Known = KnownModules.insert(std::make_pair(Path[0].first, Module)).first;