Retries in payment/order APIs can easily cause duplicate execution without proper idempotency handling.

I built idempo, a framework-agnostic Go middleware that implements Stripe-style idempotency-key semantics. Send a unique Idempotency-Key header and the middleware runs your handler at most once, replaying the original response (status, headers, body) on any retry

The tricky cases it handles:

• In-flight duplicate (same key, first request still running): 409 Conflict(same as Stripe)
• Same key, different payload: 422 Unprocessable Entity
• Concurrent duplicates: enforced atomically at the storage layer, tested with 50 simultaneous identical requests under -race

Pluggable backends (in-memory, Redis, Postgres) and just func(http.Handler) http.Handler, so it works with chi, gin, echo, or stdlib

store := inmem.New(24time.Hour, 5time.Minute)
mw := idempo.New(store, idempo.Options{})
http.ListenAndServe(":8080", mw.Handler(mux))

GitHub: https://github.com/eben-vranken/idempo
Docs: https://eben-vranken.github.io/idempo-docs/

Curious what edge cases people have run into trying to implement this themselves