Why are Non Confirmed Users able to sign up multiple times? Once a user adds their email address, why are they able to sign up a second time, and a third with the same email? Delays in the "confirm registration" email being sent is causing some users to register a second time. Then each registration code resets/expires when they attempt to sign up again. Its becomes a crazy loop.
Be sure to check that a product can only be purchased once (if your using products that is). I am not sure that would fix your problem because I have no way of knowing your setup, but I know if you have a subscription limit then that person cannot use the same account again to purchase it. Perhaps also you have come across a glitch in the system! If it is a glitch, consider putting some custom html on the sign up page urging people to email an admin instead of signing up again. Good luck!