You've correctly identified the issue; if the key existed before, then Emplace won't assign the data.

And it preserves the nice value-moving properties. I was probably overly cautious back then, seeing as push_back somehow didn't work nicely with value-moving, compared to emplace-style functions, but that's a separate issue, probably due to that function not using std::forward. But that's just a workaround trick function for template functions; it's not necessary if you just modify the ref directly by assigning it to a moved value.

Great work!