My reason for asking is I was replying to a comment to new Matt in a recent Question of his and it occurred to me that I didn't know if using @Matt would also target old Matt, which would obviously not be ideal.
I know this is late, but, yes there can be duplicate usernames. By now there are 5 Matts on this site. When replying, the system knows which users are relevant to the post (i.e. they've answered or commented already). It will pop up with their name when you type @.
I'm not sure how it handles cases when two Matts have commented on the same post. I guess that would be pretty rare.