Existing users who login with Ultimate Facebook?

Greetings everyone. I'm new here, just signing up yesterday. I have not fully explored all of the support resources available yet, but there is a question I need to ask sooner than later.

I've just installed and configured the Ultimate Facebook plugin on my website. It works great and I'm experiencing no problems, but my question is about existing users.

If an existing user logs in with Facebook, will it create a new or duplicate account for them? Or, if their FB e-mail is the same as their site e-mail, will Ultimate FB identify the logins to the one user?