I use Mac OS X Mail and have all of my work contacts in Address Book. When I receive email from one contact, the "From" field just displays his email address (as opposed to his first and last name) even though I have a contact card for him in my address book. For all other contacts, Mail displays their first and last names appropriately.
What's really weird is that if I hover over his address in the "From" field and click to bring up the context menu, there is an option for "Add to Address Book" (as if he doesn't have an entry), whereas for other contacts the option is "Open in Address Book." If I click "Add to Address Book," a new contact card is created for him even though I already have one with the same email address set up!
I've tried both assigning a first/last name to the duplicated card as well as merging the duplicated card with the original. In both cases, Mail "forgets" that the card exists and treats his email address as a new contact.
How do I get Mail to realize that there is, in fact, a contact card with his address?