I have a Exchange 2003 SP2 / Outlook 2003 SP3 / Windows XP SP2 environment. I have 40 users... ONE of my users sends a duplicate email every 4th or 5th email they send out. The receiver (internal or external) will see two of the exact same messages. However, when tracking the message in Exchange 2003 you can see that the messages in fact have different message ID's and Exchange thinks two were sent. So it appears it's not a exchange issue but an outlook issue??? Hard to say.
A few notes:
Is it virus scan? - No, this happens with the scanner on or off
New software installed on the users pc? - No, machines are locked down and nothing has been installed. Full adware and spyware check were run.
SMTP issue? - No, no one else is sending duplicates
Latest updates installed? - Yes, all updates for windows xp, exchange 2003, and outlook 2003 have been installed
Sync issue? - Doesn't appear to be, user is local with one copy of there mailbox open only... They are using cached mode with no sync errors.
POP / IMAP issue? - No, my server does not allow either.
Blackberry issue? - If it is, don't know how it's possible. We do have a blackberry server, but the messages are not being sent from the handheld, they are being sent directly from the users laptop.