Moxi Engage pulls its contacts from your email Contacts/People list and can sometimes show duplicate contacts.The first step is to login to your email and make sure that there really are no duplicate contacts.
Second, Outlook (both the software and the web app) will often LINK together duplicate contacts with the same name so you only see one. Technically, however, there are actually two. For example: one contact for the person just has an email and another just has just the mailing address.
Engage will see both since technically there are multiple contacts.
To clean these up, log into the Outlook Web App (OWA) and go to the People section. Find one of the duplicates then click "Manage" on the Contact Card and see if there is a LINK/UNLINK option when you look at them. If this is present, you need to Unlink them, then consolidate the information into just one contact. Once you've done this, you can delete the duplicate.
Unfortunately there is no easy or automatic way to do this - it has to be done one by one. However, when your contacts are cleaned up and consolidated, Moxi Engage will show the correct number of contacts!