Yes, the ability for someone to have the same email address in the user database because they logged in via Facebook and a real Q&A account, appears to be a bug. I am not sure how this is allowed either. This was addressed on another question thread, but I'm not sure it's been fixed.
I think this is an easy fix for the developer of this script. Basically, before you insert an email from the FB plugin, you check to make sure the user is not already in the database and vice versa.