I've seen this behavior before with other apps. Getting multiple messages can happen when the connection between the orginating MTA and receiving MTA is broken after the originating MTA completes message delivery and is disconnecting, but before the receiving MTA acks the disconnection request. When this happens, the originating MTA thinks the message wasn't delivered, and tries again.
One would think this kind of failure would be covered by the SMTP protocols. OTOH, it might be the specific implementation of the originating MTA involved. Never bothered to dig down further than this since the problem went away when a more reliable link was available.
