A Pub/Sub push subscription sends messages to your HTTPS endpoint, but your endpoint receives many duplicate messages. What is the most likely cause?

Pub/Sub push subscriptions require the subscriber to acknowledge messages within a defined acknowledgment deadline. If the custom endpoint fails to send an acknowledgment (ACK) for a message before this deadline expires, Pub/Sub assumes the message was not successfully processed and will re-deliver it. This re-delivery mechanism is the most common cause of duplicate messages in push subscriptions. Incorrect options: Message body size: While very large messages can cause processing delays, they don't inherently lead to duplicate deliveries unless they prevent timely acknowledgment. Out-of-date SSL certificate: An invalid SSL certificate would prevent messages from being delivered at all, not cause duplicates. Too many messages published: A high volume of messages can increase the load, but Pub/Sub is designed to handle high throughput. Duplicates arise from subscriber behavior, not just publication volume, unless it overwhelms the subscriber's ability to ACK.