Index: libcxx/include/__hash_table
===================================================================
--- libcxx/include/__hash_table
+++ libcxx/include/__hash_table
@@ -126,8 +126,8 @@
     return __n < 2 ? __n : (size_t(1) << (numeric_limits<size_t>::digits - __libcpp_clz(__n-1)));
 }
 
-
-template <class _Tp, class _Hash, class _Equal, class _Alloc> class __hash_table;
+template <class _Tp, class _Hash, class _Equal, class _Alloc, class _Traits>
+class __hash_table;
 
 template <class _NodePtr>      class _LIBCPP_TEMPLATE_VIS __hash_iterator;
 template <class _ConstNodePtr> class _LIBCPP_TEMPLATE_VIS __hash_const_iterator;
@@ -860,9 +860,13 @@
 template <class _Key, class _Hash, class _Equal>
 int __diagnose_unordered_container_requirements(void*);
 
-template <class _Tp, class _Hash, class _Equal, class _Alloc>
-class __hash_table
-{
+template <bool _Unique_Keys>
+struct __hash_table_traits {
+  typedef integral_constant<bool, _Unique_Keys> __unique_keys;
+};
+
+template <class _Tp, class _Hash, class _Equal, class _Alloc, class _Traits>
+class __hash_table {
 public:
     typedef _Tp    value_type;
     typedef _Hash  hasher;
@@ -921,6 +925,7 @@
     typedef unique_ptr<__next_pointer[], __bucket_list_deleter> __bucket_list;
     typedef allocator_traits<__pointer_allocator>          __pointer_alloc_traits;
     typedef typename __bucket_list_deleter::pointer       __node_pointer_pointer;
+    typedef typename _Traits::__unique_keys __unique_keys;
 
     // --- Member data begin ---
     __bucket_list                                         __bucket_list_;
@@ -2274,11 +2279,12 @@
                     else
                     {
                         __next_pointer __np = __cp;
-                        for (; __np->__next_ != nullptr &&
-                               key_eq()(__cp->__upcast()->__value_,
-                                        __np->__next_->__upcast()->__value_);
-                                                           __np = __np->__next_)
+                        if (!__unique_keys{}) {
+                          for (; __np->__next_ != nullptr &&
+                                 key_eq()(__cp->__upcast()->__value_, __np->__next_->__upcast()->__value_);
+                               __np = __np->__next_)
                             ;
+                        }
                         __pp->__next_ = __np->__next_;
                         __np->__next_ = __bucket_list_[__chash]->__next_;
                         __bucket_list_[__chash]->__next_ = __cp;
Index: libcxx/include/unordered_map
===================================================================
--- libcxx/include/unordered_map
+++ libcxx/include/unordered_map
@@ -1031,8 +1031,7 @@
     typedef typename __rebind_alloc_helper<allocator_traits<allocator_type>,
                                                  __value_type>::type          __allocator_type;
 
-    typedef __hash_table<__value_type, __hasher,
-                         __key_equal,  __allocator_type>   __table;
+    typedef __hash_table<__value_type, __hasher, __key_equal, __allocator_type, __hash_table_traits<true >> __table;
 
     __table __table_;
 
@@ -1921,8 +1920,7 @@
     typedef typename __rebind_alloc_helper<allocator_traits<allocator_type>,
                                                  __value_type>::type          __allocator_type;
 
-    typedef __hash_table<__value_type, __hasher,
-                         __key_equal,  __allocator_type>   __table;
+    typedef __hash_table<__value_type, __hasher, __key_equal, __allocator_type, __hash_table_traits<false>> __table;
 
     __table __table_;
 
Index: libcxx/include/unordered_set
===================================================================
--- libcxx/include/unordered_set
+++ libcxx/include/unordered_set
@@ -501,9 +501,9 @@
                   "Invalid allocator::value_type");
 
 private:
-    typedef __hash_table<value_type, hasher, key_equal, allocator_type> __table;
+  typedef __hash_table<value_type, hasher, key_equal, allocator_type, __hash_table_traits<true >> __table;
 
-    __table __table_;
+  __table __table_;
 
 public:
     typedef typename __table::pointer         pointer;
@@ -1157,9 +1157,9 @@
                   "Invalid allocator::value_type");
 
 private:
-    typedef __hash_table<value_type, hasher, key_equal, allocator_type> __table;
+  typedef __hash_table<value_type, hasher, key_equal, allocator_type, __hash_table_traits<false>> __table;
 
-    __table __table_;
+  __table __table_;
 
 public:
     typedef typename __table::pointer         pointer;