This one is a refinement for multimap/multiset.

It allows to have share the same key if managed with ref counting like 
the cow string.

     libstdc++: [_Rb_tree] Limit allocations on equal insertions [PR 96088]

     When inserting the same key several times prefer to insert the new 
entry using the
     current stored key_type instance if this copy is noexcept. 
Otherwise create a new
     key instance from input argument.

     libstdc++-v3/ChangeLog:

             PR libstdc++/96088
             * include/bits/cow_string.h 
(basic_string<>::basic_string(const basic_string&)):
             Add noexcept qualification when allocator is always equal.
             * include/bits/stl_tree.h 
(_Rb_tree<>::_M_get_insert_equal_pos_tr): New.
             (_Rb_tree<>::_M_emplace_equal_tr): New, use latter.
             (_Rb_tree<>::_M_emplace_equal_aux): New, use latter.
(_Rb_tree<>::_M_emplace_equal<_Arg>(_Arg&&)): New, use latter.
             * testsuite/23_containers/multimap/96088.cc (test01): Add 
check on redundant
             insertion.
             (test02): Likewise.
             * testsuite/23_containers/multiset/96088.cc (test01, 
test02): Likewise.

François