Mail forwarding has a bug.

If it is a multi-locational setup, and you set mail forwarding for a particular user, then the user will begin to get more than one copy of each incoming email. The number of copies that he will receive will be determined by the number of Merce mail servers the incoming message passes through. If the incoming email passes through just one server, then the user will receive only one copy. If the incoming email passes through two Merce servers, he'll receive two copies of that message. And so on.

This does not affect the number of copies he sees in his IMAP inbox because Cyrus does duplicate removal. But if you switch on mail forwarding, then the user begins to see this phenomenon in the remote mailbox to which he's forwarding his messages.