Just trying to wrap my head around this.
The first time a user tries to log into your site using their Facebook credentials, they're brought to the registration page you show. The reason for the 2 forms on this page are the following:
1) The left "Sign In" area is for if the user already has a Joomla account on your site. Signing in using this form will then connect the user's Joomla account to their Facebook account. Later, when they return, if they use Facebook to log in, it will sign them into this Joomla account. This is very useful if your site is old and has many users, they don't have to create new accounts just to be able to use some of the FB features.
2) The right side registration area is for new users to create a Joomla account to associate with their Facebook account.
In either case, I wouldn't recommend getting rid of one side or the other. While it may seem redundant with the login module, the left hand side definitely has a different purpose... and again, after the first registration, they should never see that page again.
Now that it's hopefully better explained, let us know if you're still looking to modify functionality and we can help you out.
First Time Facebook Users - Just login with facebook
