When data is imported to update contacts, CiviCRM has a set of rules to match contacts already in the database. By default, CiviCRM uses the e-mail address. If it finds a matching e-mail address it will update the matching contact. A problem will arise because not all of your contacts will have an e-mail address. So if we try to import some updates, only those contacts with an e-mail address will be updated.
In these situations we need to have a different unique identifier that we can let CiviCRM use to match records.
We will use the External Identifier field in CiviCRM as our unique identifier. This field has to have a unique value for each contact. This will provide us with a good alternative to e-mail addresses to match contact data.
Navigate to Contacts | Find and Merge Duplicate Contacts.
The screenshot shows the deduping rules that are available to use for each contact type:
If we were importing data for individuals...