The 100 Karins who really, really, really, really, REALLY love you.

Seriously though, there are so many similarities e.g.

If there’s one thing this manga does better than 100GF at this point, it’s that it got the childhood-friend angle covered early on.

I remember reading one with a similar premise years ago, where the girl was split into multiple personalities. At the end they all were absorbed by the original.