Facebook login integration has been added to the site. If you already registered before this option was enabled you will not end up with two users on the system as long as the email address you registered with is the same as your email address on Facebook.
Also please keep in mind that after login via facebook or registering your account for the first time. It does have to be approved before you will see our members features. This is a manual process, but you will get an email once this process is complete.