409 Conflict - Duplicate Record

Message:
Contact already exists. Existing ID: XXXXXXXXXXX

Cause:
Trying to create a contact (or deal, company, etc.) that already exists—usually due to duplicate email or unique custom property.

Fix:

• Use the object’s existing ID to update instead of creating a new one.
• Before creating a contact in your source system, ensure that it has an unique email addresses. A validation on the uniqueness of the email address can be enabled in the source system so duplicates are avoided. 
• Use an unique id/key for all new and existing contacts so you can ensure uniqueness across all systems ( e.g. store the Hubspot Id into the source system).