diff --git a/include/__tree b/include/__tree
index 4473ac3..15b03ec 100644
--- a/include/__tree
+++ b/include/__tree
@@ -1096,8 +1096,8 @@
     __tree(const value_compare& __comp, const allocator_type& __a);
     __tree(const __tree& __t);
     __tree& operator=(const __tree& __t);
-    template <class _InputIterator>
-        void __assign_unique(_InputIterator __first, _InputIterator __last);
+    template <class _ForwardIterator>
+        void __assign_unique(_ForwardIterator __first, _ForwardIterator __last);
     template <class _InputIterator>
         void __assign_multi(_InputIterator __first, _InputIterator __last);
 #ifndef _LIBCPP_CXX03_LANG
@@ -1332,10 +1332,7 @@
 #endif // !_LIBCPP_CXX03_LANG
 
     _LIBCPP_INLINE_VISIBILITY
-    pair<iterator, bool> __node_insert_unique(__node_pointer __nd);
-    _LIBCPP_INLINE_VISIBILITY
-    iterator             __node_insert_unique(const_iterator __p,
-                                              __node_pointer __nd);
+    pair<iterator, bool> __node_assign_unique(const __container_value_type& __v, __node_pointer __dest);
 
     _LIBCPP_INLINE_VISIBILITY
     iterator __node_insert_multi(__node_pointer __nd);
@@ -1515,8 +1512,50 @@
     _LIBCPP_INLINE_VISIBILITY
     void __move_assign_alloc(__tree&, false_type) _NOEXCEPT {}
 
-    __node_pointer __detach();
-    static __node_pointer __detach(__node_pointer);
+    struct _DetachedTreeCache {
+      _LIBCPP_INLINE_VISIBILITY
+      explicit _DetachedTreeCache(__tree *__t) _NOEXCEPT : __t_(__t),
+        __cache_root_(__detach_from_tree(__t)) {
+          __advance();
+        }
+
+      _LIBCPP_INLINE_VISIBILITY
+      __node_pointer __get() const _NOEXCEPT {
+        return __cache_elem_;
+      }
+
+      _LIBCPP_INLINE_VISIBILITY
+      void __advance() _NOEXCEPT {
+        __cache_elem_ = __cache_root_;
+        if (__cache_root_) {
+          __cache_root_ = __detach_next(__cache_root_);
+        }
+      }
+
+      _LIBCPP_INLINE_VISIBILITY
+      ~_DetachedTreeCache() {
+        __t_->destroy(__cache_elem_);
+        if (__cache_root_) {
+          while (__cache_root_->__parent_ != nullptr)
+            __cache_root_ = static_cast<__node_pointer>(__cache_root_->__parent_);
+          __t_->destroy(__cache_root_);
+        }
+      }
+
+       _DetachedTreeCache(_DetachedTreeCache const&) = delete;
+       _DetachedTreeCache& operator=(_DetachedTreeCache const&) = delete;
+
+    private:
+      _LIBCPP_INLINE_VISIBILITY
+      static __node_pointer __detach_from_tree(__tree *__t) _NOEXCEPT;
+      _LIBCPP_INLINE_VISIBILITY
+      static __node_pointer __detach_next(__node_pointer) _NOEXCEPT;
+
+      __tree *__t_;
+      __node_pointer __cache_root_;
+      __node_pointer __cache_elem_;
+    };
+
 
     template <class, class, class, class> friend class _LIBCPP_TEMPLATE_VIS map;
     template <class, class, class, class> friend class _LIBCPP_TEMPLATE_VIS multimap;
@@ -1554,13 +1593,13 @@
 // Precondition:  size() != 0
 template <class _Tp, class _Compare, class _Allocator>
 typename __tree<_Tp, _Compare, _Allocator>::__node_pointer
-__tree<_Tp, _Compare, _Allocator>::__detach()
+__tree<_Tp, _Compare, _Allocator>::_DetachedTreeCache::__detach_from_tree(__tree *__t) _NOEXCEPT
 {
-    __node_pointer __cache = static_cast<__node_pointer>(__begin_node());
-    __begin_node() = __end_node();
-    __end_node()->__left_->__parent_ = nullptr;
-    __end_node()->__left_ = nullptr;
-    size() = 0;
+    __node_pointer __cache = static_cast<__node_pointer>(__t->__begin_node());
+    __t->__begin_node() = __t->__end_node();
+    __t->__end_node()->__left_->__parent_ = nullptr;
+    __t->__end_node()->__left_ = nullptr;
+    __t->size() = 0;
     // __cache->__left_ == nullptr
     if (__cache->__right_ != nullptr)
         __cache = static_cast<__node_pointer>(__cache->__right_);
@@ -1575,7 +1614,7 @@
 //    This is no longer a red-black tree
 template <class _Tp, class _Compare, class _Allocator>
 typename __tree<_Tp, _Compare, _Allocator>::__node_pointer
-__tree<_Tp, _Compare, _Allocator>::__detach(__node_pointer __cache)
+__tree<_Tp, _Compare, _Allocator>::_DetachedTreeCache::__detach_next(__node_pointer __cache) _NOEXCEPT
 {
     if (__cache->__parent_ == nullptr)
         return nullptr;
@@ -1609,45 +1648,23 @@
 }
 
 template <class _Tp, class _Compare, class _Allocator>
-template <class _InputIterator>
+template <class _ForwardIterator>
 void
-__tree<_Tp, _Compare, _Allocator>::__assign_unique(_InputIterator __first, _InputIterator __last)
+__tree<_Tp, _Compare, _Allocator>::__assign_unique(_ForwardIterator __first, _ForwardIterator __last)
 {
-    typedef iterator_traits<_InputIterator> _ITraits;
+    typedef iterator_traits<_ForwardIterator> _ITraits;
     typedef typename _ITraits::value_type _ItValueType;
     static_assert((is_same<_ItValueType, __container_value_type>::value),
                   "__assign_unique may only be called with the containers value type");
-
+    static_assert(__is_forward_iterator<_ForwardIterator>::value,
+                  "__assign_unique requires a forward iterator");
     if (size() != 0)
     {
-        __node_pointer __cache = __detach();
-#ifndef _LIBCPP_NO_EXCEPTIONS
-        try
-        {
-#endif  // _LIBCPP_NO_EXCEPTIONS
-            for (; __cache != nullptr && __first != __last; ++__first)
-            {
-                __cache->__value_ = *__first;
-                __node_pointer __next = __detach(__cache);
-                __node_insert_unique(__cache);
-                __cache = __next;
+        _DetachedTreeCache __cache(this);
+          for (; __cache.__get() != nullptr && __first != __last; ++__first) {
+              if (__node_assign_unique(*__first, __cache.__get()).second)
+                  __cache.__advance();
             }
-#ifndef _LIBCPP_NO_EXCEPTIONS
-        }
-        catch (...)
-        {
-            while (__cache->__parent_ != nullptr)
-                __cache = static_cast<__node_pointer>(__cache->__parent_);
-            destroy(__cache);
-            throw;
-        }
-#endif  // _LIBCPP_NO_EXCEPTIONS
-        if (__cache != nullptr)
-        {
-            while (__cache->__parent_ != nullptr)
-                __cache = static_cast<__node_pointer>(__cache->__parent_);
-            destroy(__cache);
-        }
     }
     for (; __first != __last; ++__first)
         __insert_unique(*__first);
@@ -1666,33 +1683,11 @@
                   " or the nodes value type");
     if (size() != 0)
     {
-        __node_pointer __cache = __detach();
-#ifndef _LIBCPP_NO_EXCEPTIONS
-        try
-        {
-#endif  // _LIBCPP_NO_EXCEPTIONS
-            for (; __cache != nullptr && __first != __last; ++__first)
-            {
-                __cache->__value_ = *__first;
-                __node_pointer __next = __detach(__cache);
-                __node_insert_multi(__cache);
-                __cache = __next;
-            }
-#ifndef _LIBCPP_NO_EXCEPTIONS
-        }
-        catch (...)
-        {
-            while (__cache->__parent_ != nullptr)
-                __cache = static_cast<__node_pointer>(__cache->__parent_);
-            destroy(__cache);
-            throw;
-        }
-#endif  // _LIBCPP_NO_EXCEPTIONS
-        if (__cache != nullptr)
-        {
-            while (__cache->__parent_ != nullptr)
-                __cache = static_cast<__node_pointer>(__cache->__parent_);
-            destroy(__cache);
+        _DetachedTreeCache __cache(this);
+        for (; __cache.__get() && __first != __last; ++__first) {
+            __cache.__get()->__value_ = *__first;
+            __node_insert_multi(__cache.__get());
+            __cache.__advance();
         }
     }
     for (; __first != __last; ++__first)
@@ -1790,33 +1785,11 @@
         const_iterator __e = end();
         if (size() != 0)
         {
-            __node_pointer __cache = __detach();
-#ifndef _LIBCPP_NO_EXCEPTIONS
-            try
-            {
-#endif  // _LIBCPP_NO_EXCEPTIONS
-                while (__cache != nullptr && __t.size() != 0)
-                {
-                    __cache->__value_ = _VSTD::move(__t.remove(__t.begin())->__value_);
-                    __node_pointer __next = __detach(__cache);
-                    __node_insert_multi(__cache);
-                    __cache = __next;
-                }
-#ifndef _LIBCPP_NO_EXCEPTIONS
-            }
-            catch (...)
-            {
-                while (__cache->__parent_ != nullptr)
-                    __cache = static_cast<__node_pointer>(__cache->__parent_);
-                destroy(__cache);
-                throw;
-            }
-#endif  // _LIBCPP_NO_EXCEPTIONS
-            if (__cache != nullptr)
-            {
-                while (__cache->__parent_ != nullptr)
-                    __cache = static_cast<__node_pointer>(__cache->__parent_);
-                destroy(__cache);
+            _DetachedTreeCache __cache(this);
+            while (__cache.__get() != nullptr && __t.size() != 0) {
+              __cache.__get()->__value_ = _VSTD::move(__t.remove(__t.begin())->__value_);
+              __node_insert_multi(__cache.__get());
+              __cache.__advance();
             }
         }
         while (__t.size() != 0)
@@ -2325,14 +2298,15 @@
 
 template <class _Tp, class _Compare, class _Allocator>
 pair<typename __tree<_Tp, _Compare, _Allocator>::iterator, bool>
-__tree<_Tp, _Compare, _Allocator>::__node_insert_unique(__node_pointer __nd)
+__tree<_Tp, _Compare, _Allocator>::__node_assign_unique(const __container_value_type& __v, __node_pointer __nd)
 {
     __parent_pointer __parent;
-    __node_base_pointer& __child = __find_equal(__parent, __nd->__value_);
+    __node_base_pointer& __child = __find_equal(__parent, _NodeTypes::__get_key(__v));
     __node_pointer __r = static_cast<__node_pointer>(__child);
     bool __inserted = false;
     if (__child == nullptr)
     {
+        __nd->__value_ = __v;
         __insert_node_at(__parent, __child, static_cast<__node_base_pointer>(__nd));
         __r = __nd;
         __inserted = true;
@@ -2340,22 +2314,6 @@
     return pair<iterator, bool>(iterator(__r), __inserted);
 }
 
-template <class _Tp, class _Compare, class _Allocator>
-typename __tree<_Tp, _Compare, _Allocator>::iterator
-__tree<_Tp, _Compare, _Allocator>::__node_insert_unique(const_iterator __p,
-                                                        __node_pointer __nd)
-{
-    __parent_pointer __parent;
-    __node_base_pointer __dummy;
-    __node_base_pointer& __child = __find_equal(__p, __parent, __nd->__value_);
-    __node_pointer __r = static_cast<__node_pointer>(__child);
-    if (__child == nullptr)
-    {
-        __insert_node_at(__parent, __child, static_cast<__node_base_pointer>(__nd));
-        __r = __nd;
-    }
-    return iterator(__r);
-}
 
 template <class _Tp, class _Compare, class _Allocator>
 typename __tree<_Tp, _Compare, _Allocator>::iterator
diff --git a/test/std/containers/associative/map/map.cons/assign_initializer_list.pass.cpp b/test/std/containers/associative/map/map.cons/assign_initializer_list.pass.cpp
index c277e9e..f955135 100644
--- a/test/std/containers/associative/map/map.cons/assign_initializer_list.pass.cpp
+++ b/test/std/containers/associative/map/map.cons/assign_initializer_list.pass.cpp
@@ -19,10 +19,10 @@
 
 #include "test_macros.h"
 #include "min_allocator.h"
+#include "test_allocator.h"
 
-int main(int, char**)
-{
-    {
+void test_basic() {
+  {
     typedef std::pair<const int, double> V;
     std::map<int, double> m =
                             {
@@ -70,6 +70,28 @@
     assert(*next(m.begin()) == V(2, 1));
     assert(*next(m.begin(), 2) == V(3, 1));
     }
+}
+
+
+void duplicate_keys_test() {
+  typedef std::map<int, int, std::less<int>, test_allocator<std::pair<const int, int> > > Map;
+  typedef test_alloc_base AllocBase;
+  {
+    LIBCPP_ASSERT(AllocBase::alloc_count == 0);
+    Map s = {{1, 0}, {2, 0}, {3, 0}};
+    LIBCPP_ASSERT(AllocBase::alloc_count == 3);
+    s = {{4, 0}, {4, 0}, {4, 0}, {4, 0}};
+    LIBCPP_ASSERT(AllocBase::alloc_count == 1);
+    assert(s.size() == 1);
+    assert(s.begin()->first == 4);
+  }
+  LIBCPP_ASSERT(AllocBase::alloc_count == 0);
+}
+
+int main(int, char**)
+{
+    test_basic();
+    duplicate_keys_test();
 
   return 0;
 }
diff --git a/test/std/containers/associative/set/set.cons/assign_initializer_list.pass.cpp b/test/std/containers/associative/set/set.cons/assign_initializer_list.pass.cpp
index e04f49c..3762446 100644
--- a/test/std/containers/associative/set/set.cons/assign_initializer_list.pass.cpp
+++ b/test/std/containers/associative/set/set.cons/assign_initializer_list.pass.cpp
@@ -16,13 +16,14 @@
 
 #include <set>
 #include <cassert>
+#include <iostream>
 
 #include "test_macros.h"
 #include "min_allocator.h"
+#include "test_allocator.h"
 
-int main(int, char**)
-{
-    {
+void basic_test() {
+  {
     typedef std::set<int> C;
     typedef C::value_type V;
     C m = {10, 8};
@@ -36,9 +37,9 @@
     assert(*++i == V(4));
     assert(*++i == V(5));
     assert(*++i == V(6));
-    }
-    {
-    typedef std::set<int, std::less<int>, min_allocator<int>> C;
+  }
+  {
+    typedef std::set<int, std::less<int>, min_allocator<int> > C;
     typedef C::value_type V;
     C m = {10, 8};
     m = {1, 2, 3, 4, 5, 6};
@@ -51,7 +52,27 @@
     assert(*++i == V(4));
     assert(*++i == V(5));
     assert(*++i == V(6));
-    }
+  }
+}
+
+void duplicate_keys_test() {
+  typedef std::set<int, std::less<int>, test_allocator<int> > Set;
+  typedef test_alloc_base AllocBase;
+  {
+    LIBCPP_ASSERT(AllocBase::alloc_count == 0);
+    Set s = {1, 2, 3};
+    LIBCPP_ASSERT(AllocBase::alloc_count == 3);
+    s = {4, 4, 4, 4, 4};
+    LIBCPP_ASSERT(AllocBase::alloc_count == 1);
+    assert(s.size() == 1);
+    assert(*s.begin() == 4);
+  }
+  LIBCPP_ASSERT(AllocBase::alloc_count == 0);
+}
+
+int main(int, char**) {
+  basic_test();
+  duplicate_keys_test();
 
   return 0;
 }