Two seemingly identical tags:
Apple's address book client, renamed from Address Book in OS X 10.8
Apple's built-in contacts manager software for Mac OS X.
As the contacts.app tag wiki clearly states, it is a simple rename of the same thing.
Feature Request: Merge the two, and create a synonym for the other.
Edit: I'm not saying these are the only tags that need it - go ahead and merge the others as well! :)