If you using MS Outlook for accessing emails and you had merge contacts from other apps like Thunderbird or so, then you might find duplicate contacts.
Here is a simple and easy to use tool, ODIR or Outlook Duplicate Item Remover, allows you to search and find duplicate contents like emails, contacts, notes, and tasks.
This Add-In for MS Office suit is very simple to use, you just need to launch it and then select the folder for scanning and once its done it will move all items to a sub folder and then you can check them and delete.
ODIR recognizes an item as a duplicate if all of the following properties match those of another item in the same folder:
Contact items: first name, last name, company name and email address
Appointment items: subject, location, start date and end date
Task items: subject, start date, due date and status
Note items: contents of the note (Body property) and color
E-mail or Post items: – received emails: the internet message ID (this is a unique identifier for each email received) – sent emails: email subject and the time the email is sent (PR_CLIENT_SUBMIT_TIME) – unsent emails: email subject only