MS Exchange Duplicate Emails

I have been having a problem with receiving duplicate emails for a few months now. It happens randomly, but when an email is sent it will come in about 50 times each day for 5 days. Most of the emails seem to be listserv or large group emails. Does anyone know why this would be happening or have any suggestions on how to stop it? I am running NT 4.0 w/ Exchange 5.5.

Thanks
Amanda