This doesn't happen when you use the signup form but it does if you add users via the admin display and you don't first check to see if a username already exists.
There has been a fix written for this, you can find it in the FAQ under the section "Troubleshooting" or the original thread is:
Subject: Identical Usernames - Watch Out!
AstroBoy 6-Oct-00
Thread: http://gossamer-threads.com/p/108305
If the url doesn't work, please search by Subject
Unoffical DBMan FAQ
http://creativecomputingweb.com/dbman/index.shtml/
There has been a fix written for this, you can find it in the FAQ under the section "Troubleshooting" or the original thread is:
Subject: Identical Usernames - Watch Out!
AstroBoy 6-Oct-00
Thread: http://gossamer-threads.com/p/108305
If the url doesn't work, please search by Subject
Unoffical DBMan FAQ
http://creativecomputingweb.com/dbman/index.shtml/
