Committed along with the enhancement of displaying the name.

When importing data, no duplicate check occurs.  As a result, if 
importing from multiple disparate data sources, its possible to 
quickly obtain many identical entires in your addressbook.  Contact 
lists have the potential to get really messed up, because only one 
entry exists in the contact list but you have no way of knowing which 
addressbook entry it corresponds to and you may inadvertently delete 
the wrong one, if you decided to clean up your addressbook and remove 
duplicates.

The contacts API call from IMP, for adding an entry to the addressbook 
from the headers of a message, does this check (single entry only).   
This patch attempts to mimic that same behavior by checking for the 
existence of the value in the addressbook.

Room for improvement - pluck the entry that is a duplicate out of the 
import file and display the email address or name to the person 
importing the entry, instead of "Already exists".  Its too late for me 
to figure out how to do that.