I'd like to ask or suggest we have a tag developed for when we see sexual relations between: twins, clones, dopplegangers(universal counterparts). Example, I just viewed a picture of Link (Zelda) having his doppleganger penetrate him. So what tag should we use to group all these together if possible.

Interesting idea. The first things that come to mind are "look-alike", "counterpart" and "alternate_version".

"self-cest" wouldn't work since doppelgangers are different characters.

"counterpart" or "alternate_version" may work in the case of "same character from different world".

"twins" is already a tag, though "look-alike" may work if they are identical twins.

Any other ideas?

Clone_sex

That would work for clones, but what about doppelgangers? They're not really clones, and in some cases they're completely different characters.

No idea about doppelgangers, it's a huge word that not everyone uses. Twin_Sex and Clone_sex seem logical to me, no idea about doppelgangers.

What about adding "look-alike" when the characters are similar enough in appearance? That would cover many doppelgangers and otherworld counterparts. And of course, add "doppelganger" if the image actually contains one.

Just came along a tag called 'selfcest'

That's for when a character is having sex with themselves.