Here is what Microsoft suggests
http://office.microsoft.com/en-us/outlook/HP100985411033.aspx#2
But if someone has thousands of contacts then it will take care ages to to click on each duplicate and select, one might need to get a physio session to recover from those clicks. I had around 5oo duplicate entries in my contacts in Outlook because I was testing ActiveSync on an Exchange Server with my mailbox and the client I used had some default settings that duplicated all contacts.
Anyways I could recover my contact list in a minute but I thought this exercise might be helpful to someone out there.
Export your contacts to a CSV file
Import it and while importing contacts chose the following option “Do not import duplicate items
Click on next and then finish, all duplicates should be eliminated.
