It was a little unclear what the client should do when asked to send a validation email and provides a `send_attempt` value that is not greater than previous attempts. As this is intended to be for when a client mistakenly sends the request twice, it makes logical sense that the implication was to simply resend a success value so the client doesn't error even when an email may have been sent on the first attempt.
This behaviour was also mimicked in Synapse/Sydent.