Index: cfe/trunk/lib/Tooling/ArgumentsAdjusters.cpp
===================================================================
--- cfe/trunk/lib/Tooling/ArgumentsAdjusters.cpp
+++ cfe/trunk/lib/Tooling/ArgumentsAdjusters.cpp
@@ -23,14 +23,18 @@
 ArgumentsAdjuster getClangSyntaxOnlyAdjuster() {
   return [](const CommandLineArguments &Args, StringRef /*unused*/) {
     CommandLineArguments AdjustedArgs;
+    bool HasSyntaxOnly = false;
     for (size_t i = 0, e = Args.size(); i < e; ++i) {
       StringRef Arg = Args[i];
       // FIXME: Remove options that generate output.
       if (!Arg.startswith("-fcolor-diagnostics") &&
           !Arg.startswith("-fdiagnostics-color"))
         AdjustedArgs.push_back(Args[i]);
+      if (Arg == "-fsyntax-only")
+        HasSyntaxOnly = true;
     }
-    AdjustedArgs.push_back("-fsyntax-only");
+    if (!HasSyntaxOnly)
+      AdjustedArgs.push_back("-fsyntax-only");
     return AdjustedArgs;
   };
 }
Index: cfe/trunk/unittests/Tooling/ToolingTest.cpp
===================================================================
--- cfe/trunk/unittests/Tooling/ToolingTest.cpp
+++ cfe/trunk/unittests/Tooling/ToolingTest.cpp
@@ -400,6 +400,33 @@
   EXPECT_FALSE(Found);
 }
 
+TEST(ClangToolTest, NoDoubleSyntaxOnly) {
+  FixedCompilationDatabase Compilations("/", {"-fsyntax-only"});
+
+  ClangTool Tool(Compilations, std::vector<std::string>(1, "/a.cc"));
+  Tool.mapVirtualFile("/a.cc", "void a() {}");
+
+  std::unique_ptr<FrontendActionFactory> Action(
+      newFrontendActionFactory<SyntaxOnlyAction>());
+
+  size_t SyntaxOnlyCount = 0;
+  ArgumentsAdjuster CheckSyntaxOnlyAdjuster =
+      [&SyntaxOnlyCount](const CommandLineArguments &Args,
+                         StringRef /*unused*/) {
+        for (llvm::StringRef Arg : Args) {
+          if (Arg == "-fsyntax-only")
+            ++SyntaxOnlyCount;
+        }
+        return Args;
+      };
+
+  Tool.clearArgumentsAdjusters();
+  Tool.appendArgumentsAdjuster(getClangSyntaxOnlyAdjuster());
+  Tool.appendArgumentsAdjuster(CheckSyntaxOnlyAdjuster);
+  Tool.run(Action.get());
+  EXPECT_EQ(SyntaxOnlyCount, 1U);
+}
+
 TEST(ClangToolTest, BaseVirtualFileSystemUsage) {
   FixedCompilationDatabase Compilations("/", std::vector<std::string>());
   llvm::IntrusiveRefCntPtr<llvm::vfs::OverlayFileSystem> OverlayFileSystem(