Outlook 2003 Client Almost Always Sends 2 E-Mails

[Flexman]

I'm running Outlook 2003 on XP SP3 with Exchange 2003 running on SBS 2003.
One of my clients seems to almost always send a duplicate e-mail whenever
either an internal or external e-mail is sent. The client does have one rule
enabled - a 2 minute delay on outgoing mail. But even if that rule is
disabled, it still sends 2 e-mails for each "real" one sent. I've also
disabled Symantec Endpoint Management from scanning internet e-mail on the
client, but no luck.

Any ideas?