Am I missing something? I certainly took a sharp intake of breath at a question posed by the newer Iain, then panicked that the old one had decided to leave SF and had his account deleted.
Usernames are indeed not unique on Stack Exchange.
Login tokens (OpenID) are uniquely associated with an account, and accounts are unique (identified by the account number), but you can have multiple accounts with the same display name.
I couldn't have said it better than this:
People have the same names in real life. Isn't StackOverflow real life?