How do I correct a mail loop?

From time to time, a message will be delivered to a user’s inbox repeatedly (10+ times). The time stamp on the e-mail is the same on every message. There doesn’t appear to be any pattern in the sending domain, or the user who this happens to. I haven’t moved the mailboxes around. How can I correct this?

I’m running Exchange 2000 and Outlook 2000.
Thanks.