The extras one notification goes to: the stored list, re-normalized (a row written before a rule changed
still obeys today's rule), keeping only destinations that want this post type and whose channel is not
already receiving it.
taken is destination 1's channel AFTER any override redirect. An extra pointing at the same channel would
post the notification there twice, so it is dropped here rather than rejected on save - a partial save body
usually does not carry the creator's own channel, so the save could not check it.
The extras one notification goes to: the stored list, re-normalized (a row written before a rule changed still obeys today's rule), keeping only destinations that want this post type and whose channel is not already receiving it.
takenis destination 1's channel AFTER any override redirect. An extra pointing at the same channel would post the notification there twice, so it is dropped here rather than rejected on save - a partial save body usually does not carry the creator's own channel, so the save could not check it.