Here is what Microsoft suggests
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.