Basically, if `userA@domain1` has followers `userB@domain2`, `userC@domain2`, and `userD@domain2`, and they have an activity that has `to`: `followers` and `bcc`: `userD@domain2`, and domain2 has a shared inbox, you can't deliver to the shared inbox without double-delivery to userD. domain2 can do double-delivery-detection, but that kind of sucks.