The goal is exactly-once business outcomes

A message delivered twice is usually better than a message lost forever.

That's the trade-off behind most reliable messaging systems.

Retries happen. Brokers crash. Consumers crash. Networks fail. Messages get redelivered.

The goal is not exactly-once delivery.

The goal is exactly-once business outcomes.

That's why idempotency is one of the most important patterns in distributed systems.

Design for duplicate delivery.

Design for failure.