THIS IS NOT A FIX - BUT RATHER A START
Now - I tried this, but found that if you have offenders already in the system - you can not use this method to fix the problem. The database will return an error with the first offender notifying you that there are duplicate entries in the db.
If you do not have offenders - you can do it, but when you enter this - here is the error message your user gets when they try to register with a dulpicate:
A fatal error has occurred:
Failed to execute query: 'INSERT INTO gforum_User (user_last_logon,user_title,user_password,user_enabled,user_template,user_status,user_val_code,
user_registered,user_admin_validated,user_username,user_email,user_id) VALUES (?,?,?,?,?,?,?,?,?,?,?,NULL)' Reason: Duplicate entry 'email@example.com' for key 2 at GForum::User::signup_submit line 352.
Please enable debugging in setup for more details.
I am trying to identify how the error messaging works - if I get it, I will post. I think this is a needed feature and hopefully someone more knowlegable than this newbie can look at it and identify a SIMPLE fix.