My favourite example of dealing with undefined behaviour is this:
In practice, many C implementations recognize, for example, #pragma once as a rough equivalent of #include guards — but GCC 1.17, upon finding a #pragma directive, would instead attempt to launch commonly distributed Unix games such as NetHack and Rogue, or start Emacs running a simulation of the Towers of Hanoi.[7]
It's overwriting existing keys, which is fine imo. When I use a map in any language and put a new value with a new key, expected behavior is that the previous key is overwritten.