SQL error when logging into Drupal for the first time

When a FUDForum user logs into Drupal for the first time, they always receive an error similar to this:

user warning: Duplicate entry 'fudforum' for key 2 query: INSERT INTO authmap (authname, uid, module) VALUES ('fudforum', 4, 'module') in /path/to/drupal/includes/database.mysqli.inc on line 151.

In addition, I can go to the users administration and see the user listed there, but they are not assigned to any roles (such as "forum user"). When the user logs out and logs back in, they no longer see the SQL error message, and when I view them in user administration, they now are assigned the role "forum user."