Why do duplicates keep showing up in my address book?
This will most likely be from contacts duplicated on the two servers - turn off the contact service on one of the servers and you should see your dupes disappear. We had this same issue recently when migrating from an in-house Exchange server to Office 365. Various users had contacts set up on both and were complaining about having duplicates in their contacts lists. Our policy is to leave contacts stored on iCloud but you may have different priorities.