When running the notifications_use_full_name.php migration, the following error is thrown:
Duplicate entry 'notification.type.approve_post' for key 'type' 
UPDATE phpbb_notification_types SET notification_type_name = 'notification.type.approve_post' WHERE notification_type_name = 'approve_post'
The prefix for the notification's services is harcoded
This happens if the code is updated and a notification is triggered, before the database has been updated.
For me disabling the board should be the first step of the update process, so this issue is more like a won't fix.
nicofuma is right. You can't expect board to work updating files, but not database.
btw, is this the only ticket blocking 3.1-RC4 release?
While I think it's fine for errors to be thrown in such a case, we should avoid creating situations which entirely break further updates, so i do think we should fix this.