Index: llvm/trunk/lib/ToolDrivers/llvm-lib/LibDriver.cpp
===================================================================
--- llvm/trunk/lib/ToolDrivers/llvm-lib/LibDriver.cpp
+++ llvm/trunk/lib/ToolDrivers/llvm-lib/LibDriver.cpp
@@ -13,6 +13,7 @@
 
 #include "llvm/ToolDrivers/llvm-lib/LibDriver.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/BinaryFormat/COFF.h"
 #include "llvm/BinaryFormat/Magic.h"
 #include "llvm/Bitcode/BitcodeReader.h"
@@ -315,6 +316,7 @@
   }
 
   std::vector<std::unique_ptr<MemoryBuffer>> MBs;
+  StringSet<> Seen;
   std::vector<NewArchiveMember> Members;
 
   // Create a NewArchiveMember for each input file.
@@ -326,6 +328,16 @@
       return 1;
     }
 
+    // Input files are uniquified by pathname. If you specify the exact same
+    // path more than once, all but the first one are ignored.
+    //
+    // Note that there's a loophole in the rule; you can prepend `.\` or
+    // something like that to a path to make it look different, and they are
+    // handled as if they were different files. This behavior is compatible with
+    // Microsoft lib.exe.
+    if (!Seen.insert(Path).second)
+      continue;
+
     // Open a file.
     ErrorOr<std::unique_ptr<MemoryBuffer>> MOrErr =
         MemoryBuffer::getFile(Path, -1, false);
Index: llvm/trunk/test/tools/llvm-lib/duplicate.test
===================================================================
--- llvm/trunk/test/tools/llvm-lib/duplicate.test
+++ llvm/trunk/test/tools/llvm-lib/duplicate.test
@@ -0,0 +1,14 @@
+If the same file is specified more than once as an input file,
+llvm-lib should ignore all but the first occurrence of the file.
+
+RUN: rm -rf %t
+RUN: mkdir -p %t
+
+RUN: llvm-mc -triple=x86_64-pc-windows-msvc -filetype=obj -o %t/foo.o %S/Inputs/a.s
+RUN: llvm-mc -triple=x86_64-pc-windows-msvc -filetype=obj -o %t/bar.o %S/Inputs/b.s
+RUN: llvm-lib -out:%t/foo.lib %t/foo.o %t/foo.o %t/bar.o
+
+RUN: llvm-ar t %t/foo.lib | FileCheck %s
+CHECK: foo.o
+CHECK-NOT: foo.o
+CHECK: bar.o