Supporting Multiple Login Methods - How to Deal with Duplicate Usernames?

[eluser]Vik[/eluser]
A lot of sites now support multiple login methods. You can log in using your username/password from Google, Facebook, Twitter, etc.

Question: what do you do when you encounter duplicate usernames? My site has some social capabilities, so it's important for everyone to see everyone else's username. What's the correct way for me to deal with it when I've got a Bob logging in with Google credentials, another Bob logging in with Facebook credentials, and another Bob with Twitter credentials?

Thanks in advance to all for any thoughts or info.