Add User

When I attempt to add a user, either as an administrator or as the anonymous user, the email address receives the confirmation email, but the user is not added to the database. The following PHP error is logged:

Duplicate entry '13' for key 1 query: INSERT INTO users (name, pass, init, mail, status, created, changed, uid) VALUES ('sam', '6c63212ab48e8401eaf6b59b95d816a9', 'gimmy@activeresponse.org', 'gimmy@activeresponse.org', '1', '1125184324', '1125184324', '13') in /home/class/public_html/activeresponse/includes/database.mysql.inc on line 66.

I recently upgraded my site from 4.5 to 4.6, and from 4.6.0 to 4.6.3 - I dont exactly know when this happened (a potential user alerted me).

Thanks for your help.