да никто не обрабатывает 409 ошибку, достаточно 400. зачем детей слушаешь, они не в курсе
https://tools.ietf.org/html/rfc7231#section-6.5.1The 400 (Bad Request) status code indicates that the server cannot or
will not process the request due to something that is perceived to be
a client error (e.g., malformed request syntax, invalid request
message framing, or deceptive request routing).