Hi,
That is very strange. Deleting users form the db may be a factor. It would have been better to try and confirm and solve the bug about multiple user creation rather than just start deleting them. I would need to validate that really happened. If you are running multiple sites for example it would create a user for each site.
I really do not understand what can cause your current symptoms, but I would try clearing the cookies on the client machine and recycling the app pool on the server.
Hope it helps,
Joe