Choose designated Keys which ContactTalk uses to identify those contacts which are already in the database (i.e. avoid duplication).

That way, if the contact is already in the database, ContactTalk will simply update the data for that contact instead of creating a whole new contact record.

Designate specific Key(s) by highlighting the Key in the green cell and right clicking.  Then select Make Key Field.  Multiple Key fields can be set.

NOTE: You will have an opportunity to review the import in a table before it is officially merged with the database.  At that time, you can choose to go ahead and import those that ContactTalk flagged as duplicates, or choose to not import those which were not flagged.