What delivery guarantee does AWS document for Lambda event source mappings?

Why this is the answer

AWS warns explicitly that event source mappings process each event at least once and that duplicates can occur, which is why it recommends making function code idempotent. Exactly-once is the guarantee people assume and the docs deny. At-most-once would mean silently losing records, which is not the model. And per-shard ordering is about sequence, not duplication — an ordered stream can still redeliver a record.