Index: include/__hash_table
===================================================================
--- include/__hash_table
+++ include/__hash_table
@@ -909,11 +909,21 @@
         iterator __emplace_hint_multi(const_iterator __p, _Args&&... __args);
 #endif  // !defined(_LIBCPP_HAS_NO_RVALUE_REFERENCES) && !defined(_LIBCPP_HAS_NO_VARIADICS)
 
+#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
+    template <class _ValueTp>
+    _LIBCPP_INLINE_VISIBILITY
+    pair<iterator, bool> __insert_unique_value(_ValueTp&& __x);
+#else
+    _LIBCPP_INLINE_VISIBILITY
+    pair<iterator, bool> __insert_unique_value(const value_type& __x);
+#endif
+
     pair<iterator, bool> __insert_unique(const value_type& __x);
 
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
+    pair<iterator, bool> __insert_unique(value_type&& __x);
     template <class _Pp>
-        pair<iterator, bool> __insert_unique(_Pp&& __x);
+    pair<iterator, bool> __insert_unique(_Pp&& __x);
 #endif  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
 
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
@@ -1752,6 +1762,26 @@
 pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
 __hash_table<_Tp, _Hash, _Equal, _Alloc>::__insert_unique(const value_type& __x)
 {
+    return __insert_unique_value(__x);
+}
+
+
+#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+template <class _ValueTp>
+_LIBCPP_INLINE_VISIBILITY
+pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__insert_unique_value(_ValueTp&& __x)
+#else
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
+_LIBCPP_INLINE_VISIBILITY
+pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__insert_unique_value(const value_type& __x)
+#endif
+{
+#if defined(_LIBCPP_HAS_NO_RVALUE_REFERENCES)
+    typedef const value_type& _ValueTp;
+#endif
     size_t __hash = hash_function()(__x);
     size_type __bc = bucket_count();
     bool __inserted = false;
@@ -1773,7 +1803,7 @@
         }
     }
     {
-        __node_holder __h = __construct_node(__x, __hash);
+        __node_holder __h = __construct_node(_VSTD::forward<_ValueTp>(__x), __hash);
         if (size()+1 > __bc * max_load_factor() || __bc == 0)
         {
             rehash(_VSTD::max<size_type>(2 * __bc + !__is_hash_power2(__bc),
@@ -1857,6 +1887,13 @@
 #endif  // _LIBCPP_HAS_NO_VARIADICS
 
 template <class _Tp, class _Hash, class _Equal, class _Alloc>
+pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
+__hash_table<_Tp, _Hash, _Equal, _Alloc>::__insert_unique(value_type&& __x)
+{
+    return __insert_unique_value(_VSTD::move(__x));
+}
+
+template <class _Tp, class _Hash, class _Equal, class _Alloc>
 template <class _Pp>
 pair<typename __hash_table<_Tp, _Hash, _Equal, _Alloc>::iterator, bool>
 __hash_table<_Tp, _Hash, _Equal, _Alloc>::__insert_unique(_Pp&& __x)
Index: test/libcxx/containers/unord/unord.set/insert_dup_alloc.pass.cpp
===================================================================
--- /dev/null
+++ test/libcxx/containers/unord/unord.set/insert_dup_alloc.pass.cpp
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// Check that we don't allocate when trying to insert a duplicate value into a
+// unordered_set. See PR12999 http://llvm.org/bugs/show_bug.cgi?id=12999
+
+#include <cassert>
+#include <unordered_set>
+#include "count_new.hpp"
+#include "MoveOnly.h"
+
+int main()
+{
+    {
+    std::unordered_set<int> s;
+    assert(globalMemCounter.checkNewCalledEq(0));
+
+    for(int i=0; i < 100; ++i)
+        s.insert(3);
+
+    assert(s.size() == 1);
+    assert(s.count(3) == 1);
+    assert(globalMemCounter.checkNewCalledEq(2));
+    }
+    assert(globalMemCounter.checkOutstandingNewEq(0));
+    globalMemCounter.reset();
+#ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
+    {
+    std::unordered_set<MoveOnly> s;
+    assert(globalMemCounter.checkNewCalledEq(0));
+
+    for(int i=0; i<100; i++)
+        s.insert(MoveOnly(3));
+
+    assert(s.size() == 1);
+    assert(s.count(MoveOnly(3)) == 1);
+    assert(globalMemCounter.checkNewCalledEq(2));
+    }
+    assert(globalMemCounter.checkOutstandingNewEq(0));
+    globalMemCounter.reset();
+#endif
+}