Why can naive automatic retries on a Kafka producer introduce message duplication, and how does Kafka's idempotent producer fix it?

If a send succeeds on the broker side but the acknowledgment is lost in transit, a naive retry resends the same logical message, creating a duplicate on the broker. The idempotent producer (`enable.idempotence=true`) tags each message with a sequence number per producer session, letting the broker detect and silently drop an exact duplicate retry instead of writing it twice.