A client of mine just started getting duplicate email in his Outlook 2003 on his computer. Century Link mistakenly delete his account - an old qwest.net account. The have re-established it. He also upgraded his internet speed.
The only solution I have found is to Uncheck "Leave a copy of messages on the server. This will not work since he accesses his account on his phone, iPad and computer so the messages need to be left on. This is also a POP email account. This doesn't cause an issue in my Outlook 2012.
I also noticed he has 2 users, same email address but different Incoming and Outgoing mail servers. The only difference is that one is for Denver and the other Minneapolis. Not sure why this is...see no reason. Could this be causing the issue? I think it was set up this way before.
Would it help to upgrade his Outlook? Resolve the recent issue.