There are a number of Outlook plugins that can remove duplicate contacts. I've used ODIR (http://www.vaita.com/odir.asp) which is free and works fine.
More recently I've started using a simple, very quick and effective way to remove duplicate contacts that doesn't require any third party applications.
From Outlook:
1. Export (from the File > Export menu) the main contacts folder (that contains duplicates) to a Excel or CSV file. I recommend you make a backup of this folder eg ContactsBU2015
2. Select and delete all the contacts in the main folder
3. Import (from the File > Import menu) the Excel or CSV file created. Select the option: Do not import duplicate items. Select the main (empty) contacts folder as the destination.
4. Close the wizard.
The process is documented here:
http://www.msoutlook.info/question/689
PS It would be great to have a duplicate remover feature in MPE.