In case you ever get caught up in the glitch where Outlook re-downloads the entirety of someone’s pop account and now you have 2 of every email, this tool worked for me.
http://www.vaita.com/ODIR.asp
Just install it and then open Outlook. On the add-ins tab there should be a place to run it. It will move all duplicates to a subfolder of the inbox which you can delete after.
As always, back up the PST/OST before doing this.