Usernames are per-instance, though. So there could be a toofpic on another instance. I'd also add that, somewhat-annoyingly, at least on kbin, dunno about lemmy, the interface does not go out of its way to make which instance a user is on particularly clear. You have to click on their username and check. I am pretty sure that they are gonna have to change that soon, because otherwise, someone is just going to go around impersonating other people.