A couple of possibilities that might cause this to happen.
One is that you have selected the "leave a copy of messages on the server"
option in the Advanced tab of the account setup.
However, if this has just started to happen recently, the cause is likely to
be that there is a large or maybe corrupted message in the mailbox of your
ISP (or exchange server) that you are not actually downloading successfully
each time you collect the mail. When the download of all the mail messages
is not acknowledged as successful, they will not be deleted from the server
even if the "leave a copy of messages on the server" is not selected. So
each time you receive mail, you continue to receive the same messages again.
The way to fix this is to look at your mail using a web access page (if you
have that facility available) and then delete the large email through the
web page. If you don't have a web access facility, you will need to talk
with your ISP or Admin to have them look for and delete the problem message.
Hope this is helpful.
