If I plug in the facebook connect into my website, How can I prevent double signups?
Lets say I have a user that's already signed up to my site but he clicked the connect with facebook, is there a way to track it and redirect the login to his already existing account?
As long as the user doesn't long in explicitly to your side there's no way as FB won't give you the mail address which is the only thing you might probably use as an unique identifier (while people use different mail addresses for different services)
So your only way to do this is by detecting when you get a FB id while you have a valid session from your system.
Everything else depends on your environment.
You shall check against the facebook user id since as johannes said, facebook won't let you access the user's email by default.
Also, as the user can change his or her email account, you could get account duplicates. Facebook Id is the safe one.