Autonomous agents / Exactly-once execution Market: Exactly-once Python library prevents duplicate payments

exactly-once is an open-source Python library that guarantees a side effect (for example, paying an invoice) executes at most once per key and replays the stored result on subsequent calls. It provides a @once decorator / context manager, stores with different atomicity guarantees (memory, SQLite, Redis, Postgres SERIALIZABLE), crash quarantine with probers and worker leases, passthrough of provider idempotency keys via current_key(), an onchain adapter keyed by nonce, and wrappers for integrations such as LangGraph nodes and CrewAI tool runs. The project is available on GitHub under MIT and can be installed via pip.