>>747 >>748
multimapの"multi"の意味と違うなぁ。それはmapでもできる話じゃない？
cpprefjpに解説あるから調べたら？

C++ Standards だと22.2.6.4ですな。
(N4849)
An associative container supports unique keys if it may contain at most one element for each key.
Otherwise, it supports equivalent keys.
The set and map classes support unique keys; the multiset and multimap classes support equivalent keys.