This could happen if you have 2 accounts polling the same
email server. e.g on a laptop, you have 2 accounts with
different smtp settings for sending from different
locations, but both check the same pop3 incoming account.
It could also occur if you forward mail from another
account but leave a copy of messages on the server. You
may delete it from outlook, but the original remains on
the server and downloads again on the next send/receive
