GMail started with adding all the emails address to your contact list as you sent an email. Now it has improved a lot as it is bit more intelligent than before. Latest GMail feature introduce one button to merger all duplicate contacts.
This is what GMail blog recommend you
"You've been able to merge contacts one-by-one for a while, but now we've 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 don't want merged), and hit the "Merge" button."
