Kafka exactly-once semantics is just that, they are "semantics". What Kafka supports is exactly-once processing which has been supported in other stream proces

Kafka exactly-once semantics is just that, they are "semantics". What Kafka supports is exactly-once processing which has been supported in other stream processing frameworks such as Apache Storm years before Confluent's marketing. Duplicates are possible in Kafka with the current implementation of exactly-once, if one uses Kafka's consumer api it will de-dedupe on the processing side. So no, there is no such thing as exactly-once in distributed systems.