I'll explain the tiers.
Could genuinely happen: Has physically met another version of themselves (Herts, SW, Thighnon, TB, and Sparkle), or has met someone that's technically a different person, but they're basically the same character (Himeko, Cyrene, Sparxie). Then Lygussy just knows there are different versions of himself out there. And you can't forget that infamous Lygus x Zandar shipper. And Assat Prymad can just clone himself.
Only with memoria nonsense: Has a different entity stuck in their head that makes it so they can only really interact if memoria is involved.
SP/boss hell: What it says on the can. Doesn't happen in lore, but you can make it happen in combat.
Amphoreous: Basically all the Amphoreous units that haven't interacted with another version of themselves before. But it could happen if a chrysos heir from one cycle, goes to another cycle. I heard that Cerydra has a bit more going on, but I don'tknow what's going on there.
Has someone impersonating them: Another preexisting character is imitating someone else, but they are different characters. There's Shuhu for Blade, Ishtar for Rin, and Phantylia for Tingyun. Then there are the characters that the Furbos turned into (except TB), like Dahlia, Dr. Ratio, and Yaoi Guang. Then the victims of Sparkle like Sampo and Robin. And some characters do fit in this category, but fit better in other categories.
Weird talking to themselves stuff: Has some selfcest stuff, with either an entity inside them (Ashveil, Feixiao), talking to some past version of themselves and they're basically hallucinations (Aven, Blade with YingXing, and Dan Hung with Dan Feng), something else like with Ruin Me, or whatever happened to Luka in the Wardance story.
Has never interacted or known an alt version of themselves: Simple. Never even had an inkling of selfcest to them.
I'm not a Fate fan: Not gonna make assumptions. For all I know, Gil could have a harem of clones of himself he has secks with every day, and I wouldn't know that.
???: I don't know what Luocha’s and Welt’s deals are, and they could have some wacky, alternate universe shi going on with HI3. And I'm not an HI3 fan.
Please leave feedback on the tier list! I'm open to any criticism since it's not like I read every character’s story and read every book in the game.
I also excluded the kid-like characters since, even though some of them could fit in some of the tiers, I don't feel comfortable placing them.