Delivery Guarantees, Simply

• At-least-once: duplicates possible; build idempotency on consumers. (SQS Standard delivers at least once and may reorder.)
• FIFO/Exactly-once-ish: many systems try to avoid duplicates on a per-queue basis (e.g., SQS FIFO deduplication windows).
• Exactly-once processing: supported via transactions + idempotent producers in Kafka Streams/apps. Understand trade-offs.