DotNetNuke has a nice way to require unique passwords for all users. You can't simply go into the web.config, search for RequireUniqueEmail="False" and set it to RequireUniqueEmail="True". The problem is if there are already users with duplicate email addresses, bad things happen (It's been awhile since I've tried it and I don't remember specifically what happens but I know it wasn't handled well).
So, you need a way to remove the duplicate email addresses before you make the web.config change. This SQL will take care of it.