Is there any documentation that describes all of the “data integrity” validation that occurs when a contact is added or updated through the API? I have seen “Contact already exists” exceptions returned by the API when trying to add a contact with the same email as an existing contact. But I have also seen the same error when adding a contact and the email does not exist in Hubspot but the contact first and last name already exists in Hubspot. In order to integrate our current system with Hubspot, it would be best to know all validations that occur in Hubspot when a contact is added or updated.
Hey, @VBossio 
Thanks for your question. The source of truth for deduplication in HubSpot is this article – Deduplicate records in HubSpot. I’m guessing you’ve already seen this.
Can you share a simple example of a request that returns this error for a Contact that doesn’t have an email address in HubSpot? You can do this from the documentation page and copy + paste the request and response here.
Thanks! —Jaycee