To access this feature, you just have to to this:
Login to your Gmail account.
Click on your 'Contacts' (listed in the left column with compose, inbox, draft, spam, trash etc).
Click on the 'Find duplicates' button.
You will soon see a list of related email addresses, review them and uncheck the suggested unrelated contacts that you don't want to merge. Once done verifying:
Click on the 'Merge' button.
Done. Now you are good to go.