Using SBS 2003, Exchange 2003 SP2, Trend Micro Client/Server/Messaging Security 3.6. There is no pattern for when it occurs, but users occassionally receive a duplicate email from another internal recipient. If it occurs with external recipients, noone has reported it. Message Tracking is enabled on the Exchange Server. The last time it occurred, there appears to be two messages with different message id's sent to the same user from another user at the same time (though the message tracking center does not reveal the subject line). The sender only had one instance of the message in their sent items folder. Neither user has any rules that would cause a duplicate.
I have not seen this problem with other setups running the same software/versions, and the problem occurs infrequently, so I am hesitant to disable antivirus on Exchange. I am getting enough of a complaint, though, that I need to try to find a resolution. Any thoughts are greatly appreciated!