Deleted email returns… MULTIPLE times!

We are running Exchange 5.5 on win2k server and we are running outlook 2k on the client machines. the problem we are experiencing is that the clients (some NOT all) are recieving emails that have been read and deleted over and over again. Sometimes they start within an hour or so and sometimes they begin to be repeated the next day. It started with one user and now this is happening to more and more users.