At-most-once: Fire and forget. Message might be lost. Fastest but unreliable.
At-least-once: Retry until acknowledged. Message might be delivered multiple times. Most common.
Exactly-once: Message delivered exactly once. Hard to achieve. Often requires idempotency.
Most systems use at-least-once with idempotent consumers. If processing the same message twice has no side effects, duplicates don't matter. Design your consumers to handle duplicates safely.