Gmail : One button to merge all duplicate contacts

Google added feature to merge all duplicate contacts in your address book. Clicking on a button finds an groups all the duplicate contacts. All similar contacts either by name, email address or contact details are grouped together and can be meregd in a snap.

One more convincing reason to use gmail/google contacts.

More details here http://gmailblog.blogspot.com/2009/12/one-button-to-merge-all-duplicate.html