
Personally, I think because the novel setting is a cheap ABO bl, all omega-bottoms look alike (brown hair, teary-eyed, cute, girly) and all alpha-tops look alike (handsome, serious, dominant, rich CEO type) which could explain why MC's husband, ML and future-ML (Gwak Hyeol) look identical, and MC and future-MC (Baek Kkotsu) look identical.

The premise of this story is that a kindergarten teacher is transmigrated into a bad harlequinesque novel written by his sister as the dead mother of the black-belly top. He barely has a few paragraphs in the original novel as a depressed widow who was abused by his in-laws and unalived himself, leaving behind a son who eventually grew up to be the the crazy obssessive yandere alpha (ML of the novel).
It's quite possible that the reason yandere top fixated on the MC/omega of the novel is BECAUSE he looks like his dead mother. Speaking as a veteran consumer of harlequins and cheap paperback bodice rippers.
I just read the raws… so here’s some spoiler.
I saw a comment who said that the child might be switched at birth, well, i doubt it bcs on chap 12 ML said that the woman on the photo was his older sister who died bcs of illness and he adopted her baby (kkotsu) as his son. on chap 14 there’s also MC’s bro in law who came to bring MC’s son (hyeol) back from the kindergarten and his appearance, well, his hair and eye color are the same as hyeol. plus on chap 18 MC’s dead husband appeared in a slight flashback, can’t see the face tho, but he has the same hair color as hyeol. I have yet to read the rest of the raws, currently, there’re 32 chaps for the raws so we’re far behind… my korean isn’t that good so i hope there’ll be someone who’s willing to pick up this series bcs it seems interesting and the children are just so cute esp hyeol (he’s sooo jealous of kkotsu because he just want to hog his father all to himself>.<).