NoMoreDupes for Outlook' is an Outlook add-in to clean your Outlook duplicates. Not only are your contacts and emails supported, but all items within Outlook. You can even search across folders. Perhaps you have the same contact in different folders. Search for it - find it - delete it.
You can get rid off your duplicates in seconds even if you have a big Outlook data file. You may customize the way NoMoreDupes recognizes doubled up entries: should NoMoreDupes look only at first name or last name or additionally on the company name?