Windows only: Outlook Duplicate Items Remover, a free Windows utility, does its namesake job pretty well, explains how it does it, and gives you the option of undoing its changes, making it a worthy tool for a tiring task. Simply install ODIR, activate it from Outlook’s toolbar, and point it at a mail folder. The app finds anything it thinks might be a duplicate, then moves it into a marked sub-folder, which you can look through, back up, or simply delete. Easy enough, and much easier than brain-wracking filters or other such dupe-deleting methods. Outlook Duplicate Items Remover is a free download for Windows systems only.

Outlook Duplicate Items Remover - http://www.vaita.com/ODIR.asp