This error has been reported by users of the Board Announcements extension, but they also report it happening with the Contact Admin Page. Both use the db_text class and it's set_array() method.
Basically, they get an error like: Duplicate entry 'contact_admin_info' for key 'PRIMARY' 
It looks like the set_array() method in the db_text class is trying to do an insert, when it should be doing an update, possibly because the sql_affectedrows is not evaluating as expected. These users are experiencing the error when submitting the form, which should perform an update.