Index: lib/Tooling/JSONCompilationDatabase.cpp
===================================================================
--- lib/Tooling/JSONCompilationDatabase.cpp
+++ lib/Tooling/JSONCompilationDatabase.cpp
@@ -251,20 +251,44 @@
   return Arguments;
 }
 
+static std::vector<std::string>
+deduplicateCommandLine(const std::vector<std::string> &Nodes) {
+  // This method filters out flags like:
+  //   '-MD', '-MMD', '-MG', '-MP', '-MF', '-MT', '-MQ'
+  // Or might transform flags too like:
+  //   '-Dkey=value' to ['-D', 'key=value']
+  //   '-I.' to ['-I', '.']
+  return Nodes;
+}
+
+struct CompareCompileCommand {
+  bool operator()(const CompileCommand &Lhs, const CompileCommand &Rhs) const {
+    return (Lhs.Directory == Rhs.Directory) &&
+           (Lhs.Filename == Rhs.Filename) &&
+           (Lhs.CommandLine == Rhs.CommandLine) &&
+           (Lhs.Output == Rhs.Output);
+  }
+};
+
 void JSONCompilationDatabase::getCommands(
     ArrayRef<CompileCommandRef> CommandsRef,
     std::vector<CompileCommand> &Commands) const {
+  // To filter out duplicate elements put the result first into a set.
+  std::set<CompileCommand, CompareCompileCommand> Results;
   for (int I = 0, E = CommandsRef.size(); I != E; ++I) {
     SmallString<8> DirectoryStorage;
     SmallString<32> FilenameStorage;
     SmallString<32> OutputStorage;
     auto Output = std::get<3>(CommandsRef[I]);
-    Commands.emplace_back(
+    Result.emplace(
         std::get<0>(CommandsRef[I])->getValue(DirectoryStorage),
         std::get<1>(CommandsRef[I])->getValue(FilenameStorage),
-        nodeToCommandLine(Syntax, std::get<2>(CommandsRef[I])),
+        deduplicateCommandLine(
+          nodeToCommandLine(Syntax, std::get<2>(CommandsRef[I]))),
         Output ? Output->getValue(OutputStorage) : "");
   }
+  // Copy the unique elements into the real return value.
+  std::copy(Results.begin(), Results.end(), Commands.begin());
 }
 
 bool JSONCompilationDatabase::parse(std::string &ErrorMessage) {