Duplicate emails

Hi,

Ever since I have had my iMac and using Mail (approx 18 months) I constantly get duplicate emails. That is, once the emails have downloaded from my ISP server to my Mail, I delete whatever I don't want. A few minutes later that some lot of emails is downloaded again. I repeat the steps of deleting. This can happen up to three or four times at a time. I have also checked my Trash and its not duplicated there even though I have deleted it three or four times. In my Advanced section, the box is ticked to have emails removed from server immediately and this has always been the case.

Please, can anyone provide a solution to this problem.

Thanking you in advance.