this seems so messed up. I like kbin, don’t get me wrong, but I consider this to be a bug, not a feature. When you have upvotes and downvotes one next to the other, you (a user) expect these 2 to do the exactly opposite action. Not one of them just add something in your favourites while the other starts negating another user’s karma.
I don’t think this is realistic at all. It breaks the current philosophy of the fediverse where each instance can be both autonomous and federated. What would happen if for example an instance wanted to federate after they already had a couple accounts. Would they need to delete these users because the username exists? This is the reason that the second part (after the “@”) exists.
Also look at the email. Ofcourse it is possible to have the same name with users in other email services. It would be very weird not to be allowed to get the yourname@yourfavoriteservice.com because the yourname@anotherservice.com already exists.
What you are suggesting introduces and requires a central authority that would be responsible for that, but this again, breaks the philosophy of the fediverse itself.