Duplicate results with rebalance storm

A Kafka consumer for the fraud-scoring topic (read_committed, 16 partitions, 8 instances) feeds an external scoring API and produces results to an output topic. At 22:30 the output topic shows a flood of duplicate score results, the input consumer group's commit-rate JMX metric drops to near zero, and rebalance-rate is firing continuously; lag on the input topic is rising even though the instances are busy at ~80% CPU. Recent context: the scoring API's latency degraded at 22:20, a single record's process() now sometimes takes 45s. The consumer's max.poll.interval.ms is 30000 (30s) and offsets are committed only after each batch finishes. Triage and explain the duplicate-storm and the stalled commits.