The Salesforce outbound message was failing on the Salesforce side because no acknowledgment came back, so Salesforce kept retrying. Fix it in Integromat by adding a Webhook Response step right after the webhook that returns status 200 with a SOAP acknowledgment envelope in the body. Once Salesforce gets the ack, the redeliveries stop.

Context: Stack Overflow question: an Integromat scenario starting with a custom webhook fed by Salesforce keeps re-running the same data every 2 hours; the webhook queue refills with duplicate payloads.