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' [1062]

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.

https://www.phpbb.com/community/viewtopic.php?f=466&t=2269581
https://www.phpbb.com/community/viewtopic.php?f=466&t=2269036