Outlook Duplicate Items Remover (ODIR)
The free Outlook Duplicate Items Remover solves the problem handily. It adds a new Outlook menu option called ODIR that will remove duplicates for you. Click the ODIR menu option, then choose Remove Duplicate Items, and select a folder where you want to remove duplicate items. Then click Remove Duplicate Items, and it looks for all duplicates in the folder. It thens kills the duplicates, but it also keeps a backup, in case you want to restore any.