What is the difference between exactly-once execution and idempotency in durable execution systems like Diagrid Catalyst?

Exactly-once execution means a workflow step runs precisely one time, even after failures. Durable execution guarantees at-least-once delivery; your external side effects must be idempotent to avoid duplicates. Idempotency ensures repeated calls produce the same result. Design activities with idempotency keys or reconciliation logic. Catalyst persists state and replays steps, but idempotency remains your responsibility.