I am receiving this type of error message.

"message": "Fail State with Cause: Contact ID 0033400001rCgXDAA0 failed to sync :: Record validation errors :: Failed to create/update Zendesk record :: details: map[external_id:[map[description:External 0033400001rCgXDAA0 has already been taken error:DuplicateValue]]]."

I would expect that it would only attempt an update and not a create. Part of this is if the Account Name changes but there is already an external id that is mapped to the SFDC record where this causes confusion I believe.