Re-receiving old emails


I don’t know much about computers and the internet. I have all my emails forwarded to entourage. Every once in a while all of my old emails that I’ve already received and checked are re-sent to me. However, this week I’ve been receiving old emails every day! My computer tech guy said that the computer’s fine and that it might have to do with the server. Any ideas of why this is happening and how to fix it?? Thank you!