When searching for duplicate contacts to merge with, you can only search by either Name, Organisation and Email address so one or more of these details should be completed in order to find the duplicate contact record to merge with.

Please see the Merge Contacts window below - this shows that you can enter into the search field either the contact's name, organisation name or email address in order to find matching duplicate contacts: 

When a match is found - as shown above put a check in the box for the contact you wish to 'merge' into the main 'master' contact record then click on the Merge button as shown above.