Seems you have a major problem which I suspect cannot be solved. Why not switch to another email program?
When my email has an attachment multiple copies are sent, with the original remaining in the Outbox. I do not know if there is a maximum number sent as a recipient always sends me an email alerting me to the multiple messages, and I abort Outlook. I suspect the attachment needs to be of a reasonable size before the problem arises.
I replaced my Symantec Firewall and Virus Checker with Trend Micro PC-cillin, but this didn't solve the problem.
I know of another user who has a similar problem, in that I received 17 copies from him yesterday.
I use XP Home and Outlook, with software up to date. My PC has a router so I can easily plug my laptop into the Interent without disconnecting the desktop.
HELP

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic