... the origin server MUST respond with either a) the 412 (Precondition Failed) status code or b) one of the 2xx (Successful) status codes if the origin server has verified that a state change is being requested and the final state is already reflected in the current state of the target resource (i.e., the change requested by the user agent has already succeeded, but the user agent might not be aware of it, perhaps because the prior response was lost or a compatible change was made by some other user agent). In the latter case, the origin server MUST NOT send a validator header field in the response unless it can verify that the request is a duplicate of an immediately prior change made by the same user agent.
– https://tools.ietf.org/html/rfc7232#section-3.1
– https://tools.ietf.org/html/rfc7232#section-3.1