I've got wordpress and PHPBB forums on the same server, and I've got the plugin setup to work most of the time (users can sign in or out either via WP or on the forums, and they'll be automatically logged in/out on the other; password changes affect both login tables, etc). However, sometimes when trying to log in a user gets an error message about:
Duplicate entry '(their username)' for key 'username_clean' 
and I'm trying to figure out why that's happening and how to stop it. My guess so far is that for some reason the script is trying to add the user to the database again, but since the user is already registered, trying to insert the username fails since there is already one in the same table. At the bottom of the error page, there's a note suggesting to contact the board administrator, which makes me feel like this could be with the phpBB side of things.
Any tips about where to start would be great, even just which files generate those error messages would be a good start.