I tested several other apartments, including one of the penthouses (1 Torendi Tower Penthouse), the single apartment (888 Spire Apartments), and The Old Salt House. I got the same results at all of them. The mailslot assigned to the apartment/lot causes the action to queue twice. Mailslots assigned to other apartments do not.
I was able to add a standard mailbox to the Old Salt House along with another mailslot. Both mailslots caused the action to queue twice but the standard mailbox only queued it once.
I also tested some houses in other worlds. They all had standard mailboxes and the action only queued once.
At this point, I think the bug is limited to the mailslots.
