Why do duplicates keep showing up in my address book?

I maintain a very clean address book (OSX), complete with photos and profile info of my contacts. Contacts are on iCloud and Exchange.

However, sometimes I'll get an email from a contact and the picture won't show up. In fact, an entirely new email-entry shows up in the address book even if that email is already associated with a contact.

Any advice on how I can make this stop happening? My Contacts is now full of duplicates.