Index: llvm/lib/TableGen/Main.cpp
===================================================================
--- llvm/lib/TableGen/Main.cpp
+++ llvm/lib/TableGen/Main.cpp
@@ -73,7 +73,7 @@
                                   EC.message() + "\n");
   DepOut.os() << OutputFilename << ":";
   for (const auto &Dep : Parser.getDependencies()) {
-    DepOut.os() << ' ' << Dep.first;
+    DepOut.os() << ' ' << Dep.first();
   }
   DepOut.os() << "\n";
   DepOut.keep();
Index: llvm/lib/TableGen/TGLexer.h
===================================================================
--- llvm/lib/TableGen/TGLexer.h
+++ llvm/lib/TableGen/TGLexer.h
@@ -19,7 +19,6 @@
 #include "llvm/Support/DataTypes.h"
 #include "llvm/Support/SMLoc.h"
 #include <cassert>
-#include <map>
 #include <memory>
 #include <string>
 
@@ -87,10 +86,11 @@
   unsigned CurBuffer = 0;
 
 public:
-  typedef std::map<std::string, SMLoc> DependenciesMapTy;
+  typedef StringSet<> DependenciesSetTy;
+
 private:
   /// Dependencies - This is the list of all included files.
-  DependenciesMapTy Dependencies;
+  DependenciesSetTy Dependencies;
 
 public:
   TGLexer(SourceMgr &SrcMgr, ArrayRef<std::string> Macros);
@@ -99,7 +99,7 @@
     return CurCode = LexToken(CurPtr == CurBuf.begin());
   }
 
-  const DependenciesMapTy &getDependencies() const {
+  const DependenciesSetTy &getDependencies() const {
     return Dependencies;
   }
 
Index: llvm/lib/TableGen/TGLexer.cpp
===================================================================
--- llvm/lib/TableGen/TGLexer.cpp
+++ llvm/lib/TableGen/TGLexer.cpp
@@ -379,15 +379,7 @@
     return true;
   }
 
-  DependenciesMapTy::const_iterator Found = Dependencies.find(IncludedFile);
-  if (Found != Dependencies.end()) {
-    PrintError(getLoc(),
-               "File '" + IncludedFile + "' has already been included.");
-    SrcMgr.PrintMessage(Found->second, SourceMgr::DK_Note,
-                        "previously included here");
-    return true;
-  }
-  Dependencies.insert(std::make_pair(IncludedFile, getLoc()));
+  Dependencies.insert(IncludedFile);
   // Save the line number and lex buffer of the includer.
   CurBuf = SrcMgr.getMemoryBuffer(CurBuffer)->getBuffer();
   CurPtr = CurBuf.begin();
Index: llvm/lib/TableGen/TGParser.h
===================================================================
--- llvm/lib/TableGen/TGParser.h
+++ llvm/lib/TableGen/TGParser.h
@@ -129,7 +129,7 @@
   bool TokError(const Twine &Msg) const {
     return Error(Lex.getLoc(), Msg);
   }
-  const TGLexer::DependenciesMapTy &getDependencies() const {
+  const TGLexer::DependenciesSetTy &getDependencies() const {
     return Lex.getDependencies();
   }
 
Index: llvm/test/TableGen/duplicate-include.inc
===================================================================
--- /dev/null
+++ llvm/test/TableGen/duplicate-include.inc
@@ -0,0 +1,8 @@
+
+#ifndef DUPLICATE_INCLUDE
+#define DUPLICATE_INCLUDE
+
+// This is used by the duplicate-include.td test
+def InInclude;
+
+#endif
Index: llvm/test/TableGen/duplicate-include.td
===================================================================
--- /dev/null
+++ llvm/test/TableGen/duplicate-include.td
@@ -0,0 +1,6 @@
+// RUN: llvm-tblgen -I %p %s | FileCheck %s
+
+// CHECK: def InInclude
+
+include "AllowDuplicateInclude.inc"
+include "AllowDuplicateInclude.inc"