Combining Duplicate Users

We have users with duplicate accounts. This was brought to my attention when a user had an account already, and then signed up "using Facebook", resulting in her having 2 accounts on our CC site. She signed up for a volunteer opportunity with the "Facebook" account and failed to remember because that account is registered with an email she doesn't really use.

The only thing I could figure out was deleting the account she signed up using Facebook. It was no problem because she hadn't submitted any hours with that account, so no major data was really lost. Is there a way to combine the duplicate accounts? I would like it if there was a "Duplicates" notification sections in the back Admin side so I could review possible duplicate user accounts. I also want to ensure all users are signing up for volunteer opportunities, getting all notifications and receiving their service hours under their own (1) respective accounts.