User warning: Duplicate entry

I've ran this numerous times and the error is this: If a new user registers in a normal fashion and the check boxes for phplist are checked, the following error occurs for each list item even though the user isn't registered in the phplist database.

user warning: Duplicate entry '18-1' for key 1 query: INSERT INTO phplist_listuser(userid, listid, entered) VALUES(18, 1, NOW()) in /hsphere/local/home/wingandaprayer/a-wing-and-a-prayer.org/sites/all/modules/phplist/phplist.module on line 622.

However, if the boxes are unchecked, the registration process continues without error. Phplist is updated with the user email but does not subscribe the user to any list. From all indications, the program seems to work correctly, but the errors at the beginning of the process doesn't help to build user confidence.