When creating a sync between PhoneBurner and Hubspot, the email address is used as the identifying field from Hubspot prevent duplicates, but Hubspot does not require this option.
Hubspot users can choose if they want to limit their syncing to only sync contacts with an email address.
We advise that you should only choose to only sync records that have an email address, otherwise you run the risk of having Hubspot create duplicates in both system and it can be difficult to sort out.
What can happen is , when a record is by Hubspot created on the PhoneBurner side through the data sync, the Hubspot system would see this record in PhoneBurner (which it just created) and create a new record in Hubspot back in Hubspot. Since there was now a new record in Hubspot the Datasync would then create another record in PhoneBurner, which then caused Hubspot to create another new record.