Created by: mlutfy
I ran into #3 (closed) and could reproduce it fairly systematically:
For the problem to happen, activity creation must be enabled (which is the CiviCRM default, iirc).
The interface would show "DB error: already exists", this is because of
writeToDB which checks if an activity already exists for the mailing, and if so, it adds the contact as a target to that activity. If they already received the mailing, then it causes a duplicate, which the DB prevents because of unique indexes.
And then, for some mysterious reason which could only be found in the forgotten depths of old BAOs, CiviCRM resends the mailing to all the original recipients of the mailing.