How is it possible?

[R]

Looking at the list of today's birthdays, there are two users by the name of "pmoloney" who have their 29th birthday today. Obviously this is the same user, but how can this person have managed to register twice with exactly the same name - I thought this was physically impossible? (There are no spaces before or after the name - I've checked.)

Or maybe the account was duplicated when he/she registered? Both accounts were registered on 18 June, and both have a small (but not the same) number of posts since then.