Multiple emails keep repeating

I have a client on Win98, using Outlook2000 Client, a stand a lone system. One email, for about a month keeps coming in over and over again, 100 of times. I thought it was a virus or problem from the sender, however, not the same problem has started with a few other emails, it arrives 4 at a time the same email, and keeps arriving about 4 at a time, over and over again, I will soon hace 1000’s of the same emails. I am using norton, and it up to date.

Any ideas?