I forgot the title

It's about recessive omega uke was a replacement for his twin brother (dominant omega) who eloped/ran away with the uke's boyfriend. Twin was supposed to be engaged with the seme but since uke was a replacement he had no choice but to pretend he was the dominant uke until the twin comes back. The seme eventually finds out or actually knew it's not his fiancee and it's a recessive but he's fine with bec he kinda obsesses with uke. It's one of the mangas where you think it'd be toxic but it's actually kinda fluff.