I just encountered a good answer:
which was the first post by a new user (peter). I thanked him for his good first answer, and he objected to my characterization of it as his first. He says he has had "many" accounts; each time he's prompted for his username and password he registers again.
I can't find any of his other accounts (there's no way to search by the username "peter"), and I have no evidence that he's doing any harm. I am a bit ticked that he snitted at me for treating him as a newbie, but that's my problem, not SE's.
Is there anything to be done here, or should I just continue to treat people as their profiles report?
Edit: I've been corrected; you can search users by name, although it's a substring search; "peter" returns 300+ names.