By default, the WPCRM_System_Create->contacts() method checks to see if a contact’s name exists in your CRM before determining whether to create a new contact or update an existing contact.

The wpcrm_system_create_contacts_email_key filter provides you with the option to instead use the contact’s email address for this criteria.

The filter by default returns false. Making it return true (or any other value) will allow you to check to see if a contact’s email address exists before creating or updating an existing contact.