It's a romance where a guy confesses to a girl with a boyfriend, and he is rejected.

However, the teacher thinks it's a good chance to experiment, and so he "clones" the girl, and has one live with the rejected boy, while the other girl stays with her boyfriend.

After a period of time, the clones will merge back. The competition is to see which clone's feeling for their respective guy is stronger. Whoever's feelings are stronger, the merged girl will end of liking that person in the end.

It's a bit vague and I think it doesn't have many chapters out, but I can't recall the name.

It is probably Uwagaki.

Ah thank you, that's exactly what it was.

You are welcome!