encountered an issue a few months ago where a couple of users reported duplicate messages in their unity mailboxes. i verified that this was indeed the case. same exact time stamp and same content. i looked around for a solution and found something on a forum that suggested restarting the Connection MTA service. this service is only active on the server currently running as primary.

so, i went to restart the service and the system reported that the server must perform a failover for the service to be restarted. then i figured if i must do a failover i might as well reboot the server completely. after reboot the issue went away. this was unity connection 8.5 SU2.