I keep a copy of messages on the server myself and I can't say that I have ever received old messages, again, unless I deliberately made it so.
Though every few months, I remove that setting and download everything off the server.
Then I reinstate the "Leave a copy of messages on the server" option.
How often does Outlook do a Send/Receive? If it is less than 10 minutes, you might consider reconfiguring the e-mail client to do a Send/Receive at 10 minutes intervals.
Anti-virus software can also be a cause, see this link for more ideas:
http://www.howto-outlook.com/faq/duplicates.htm