Managing a big address book can be a challenge, so it i;s no surprise that the top request for Google contacts is a fast, easy way to merge duplicate contacts. You have been able to merge contacts one-by-one for a while, but now we have added a single button that merges all your duplicate contacts at once. To clean up your contact list in one fell swoop, just click the “Find duplicates” button in the contact manager, review the merge suggestions (and uncheck any suggestions you do not want merged), and hit the “Merge” button.

via Official Gmail Blog.