This link shows how to do it.
Here is an excerpt from that article:
- In Contacts, select the contacts folder that has duplicate contacts.
- In the Navigation Pane, under Current View, click Phone List. This is the best view to scan your contacts list and see the duplicate contacts. Now you can sort the list by modified date and group the duplicates together.
- On the View menu, point to Current View, and then click Customize Current View.
- Click Fields, select Modified in the Available fields list, and then click Add.
- Click Move Up until Modified is at the top of the Show these fields in this order list.
- Click OK twice.
- In the list of contacts, hold down CTRL while you select each duplicate contact.
- When you have selected all the duplicate contacts, press DELETE.