Ultimate Facebook usage clarification

Looking at using this plugin on a site but I have a question.

When someone is already a user on the site, what is the process that will allow them to login using facebook which will link it to their existing account? Every time I try it I end up with a new user created which is of course not the way we want things to go.

I have used facebook connect on other sites in other ways and haven’t had this issue but I have always coded this functionality in manually.

I would like to make the use of this feature as painless as possible for new users and especially existing users.