I've had a few instances of two copies of a VMAIL message being received, one right after the other. The only difference was the SNR shown in the message.
While I don't really know what might have caused the reception of the duplicate, I observed a duplicate SEND from my station, which might explain it.
I had a VMAIL send fail due to QSB. Awhile later, I connected again, and saw that the previously unsent VMAIL text appeared in the VARA-HF buffer (ready to be sent) and ALSO appeared in the "pending" buffer, where it would have been sent again.