mail can't recognize my contact email address

While I can compose and have Mail fill in an address from address book, Mail often fails to recognize an incoming email address as already being in Address Book, and so spells out the whole name and email address in the From field. The contextual menu offers to Add to Address Book. But it's already there. If I opt to Add to AB, it'll make a new contact, a dupe, in other words. Anyone have a fix for this?