Remove duplicate items from Outlook with ODIR
ODIR is a free Windows utility that integrates itself with Outlook. Once installed, you should notice a new ODIR menu in Outlook. When you click Remove Duplicate items, you can choose any Outlook folder to scan. ODIR can handle contacts, calendar items, tasks, notes, or emails.
The utility scans contacts to find identical first and last names, email addresses and company names. If you've got two items that are similar, but not identical, ODIR may still try to move one to a new folder. Becuase the program isolates your duplicates instead of deleting them, you can always go into the new folder and move them back.