Jobs transcode multiple times simultaneously

An SQS-driven video-transcoding worker pool alerts at 03:00: the same jobs are being processed by multiple workers simultaneously, transcodes are running 2–3x, and S3 shows duplicate output objects. Dashboards: NumberOfMessagesReceived is ~2.5x NumberOfMessagesDeleted; the queue's ApproximateAgeOfOldestMessage is fine. The queue's VisibilityTimeout is 30s. Recent context: a new codec was rolled out yesterday that pushed average transcode time from ~20s to ~110s. There's no DLQ configured and maxReceiveCount is high. Triage and mitigate this redelivery storm.