Idempotency key for expensive POST requests

Is your feature request related to a problem? Please describe.
Clients retrying POST actions is bad especially when it's expensive like in documents.

Describe the solution you'd like
To overcome processing expensive duplicate requests, we can introduce an idempotency_key in requests which clients can use to retry. This idempotency_key needs to be stored against the relevant entity, so when users/clients retries creating the same entity, entity creation can be skipped and the API can just return the already existing entity.

Additional context
See Stripe's API docs - https://docs.stripe.com/api/idempotent_requests

El issue no especifica archivos, pruebas ni puntos de entrada. Empieza localizando los handlers POST y el modelo de persistencia de la entidad relevante; después, lee la documentación de Stripe sobre solicitudes idempotentes. Se considera terminado cuando los reintentos duplicados con la misma clave omiten la creación de la entidad y devuelven la entidad existente.