idempotency key
A unique label a client sends with a request so the server can recognise a retry and not act twice.
An idempotency key identifies a client's intended operation; the server stores the outcome under the key and returns it again for a repeat with the same key and request, and rejects the key reused for a different request.
Examples
The phone sends
POST /commandswithIdempotency-Key: k-7; when the202is lost and it retries with the same key, the server recognises the retry and doesn't run the command twice.
See also