X-Request-Id. That header is the only correlation identifier. The JSON body does not include request_id.
Statuses
Commercial
duplicate_not_accepted is not HTTP 409. It arrives asynchronously on the status object after a 202 create.
Support
Send theX-Request-Id response header, submission_id, external_reference, and environment. Do not send the raw request body in tickets that may be forwarded to shared channels.