Index: include/llvm/Support/TarWriter.h
===================================================================
--- include/llvm/Support/TarWriter.h
+++ include/llvm/Support/TarWriter.h
@@ -13,6 +13,7 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/raw_ostream.h"
+#include <set>
 
 namespace llvm {
 class TarWriter {
@@ -20,12 +21,13 @@
   static Expected<std::unique_ptr<TarWriter>> create(StringRef OutputPath,
                                                      StringRef BaseDir);
 
-  void append(StringRef Path, StringRef Data);
+  bool append(StringRef Path, StringRef Data);
 
 private:
   TarWriter(int FD, StringRef BaseDir);
   raw_fd_ostream OS;
   std::string BaseDir;
+  std::set<std::string> Files;
 };
 }
 
Index: lib/Support/TarWriter.cpp
===================================================================
--- lib/Support/TarWriter.cpp
+++ lib/Support/TarWriter.cpp
@@ -169,10 +169,15 @@
     : OS(FD, /*shouldClose=*/true, /*unbuffered=*/false), BaseDir(BaseDir) {}
 
 // Append a given file to an archive.
-void TarWriter::append(StringRef Path, StringRef Data) {
+bool TarWriter::append(StringRef Path, StringRef Data) {
   // Write Path and Data.
   std::string Fullpath = BaseDir + "/" + sys::path::convert_to_slash(Path);
 
+  bool Exist;
+  std::tie(std::ignore, Exist) = Files.insert(Fullpath);
+  if (Exist)
+    return false;
+
   StringRef Prefix;
   StringRef Name;
   if (splitUstar(Fullpath, Prefix, Name)) {
@@ -192,4 +197,6 @@
   OS << std::string(BlockSize * 2, '\0');
   OS.seek(Pos);
   OS.flush();
+
+  return true;
 }