During Contact creation, the following validations are performed:
Contact name validation:
It's checked whether there is already a Contact record with the same Contact name in the Xero Contacts. If this validation fails - an error pop-up will prompt you to change the Contact name.
Contact duplication:
If two users are creating a new Contact with the same name, only the first user will be successful. The second user will receive an error message saying that a Contact with this name already exists: