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Can't be problem of the sub
D. Your custom endpoint is not acknowledging messages within the acknowledgement deadline Why This Causes Duplicate Messages How Pub/Sub Delivery Works: Pub/Sub delivers a message to your HTTPS endpoint Your endpoint must return an HTTP success status (200, 201, 202, or 204) within the acknowledgement deadline (default is 10 seconds) If Pub/Sub doesn't receive acknowledgement in time, it assumes delivery failed Pub/Sub redelivers the message, causing duplicates
D Why are there too many duplicate messages? Pub/Sub guarantees at-least-once message delivery, which means that occasional duplicates are to be expected. However, a high rate of duplicates may indicate that the client is not acknowledging messages within the configured ack_deadline_seconds, and Pub/Sub is retrying the message delivery. This can be observed in the monitoring metrics pubsub.googleapis.com/subscription/pull_ack_message_operation_count for pull subscriptions, and pubsub.googleapis.com/subscription/push_request_count for push subscriptions. Look for elevated expired or webhook_timeout values in the /response_code. This is particularly likely if there are many small messages, since Pub/Sub may batch messages internally and a partially acknowledged batch will be fully redelivered.