I've implemented DNT Captcha in dot net 3.1 and it works well on front end. But there is one issue. I've submited form where captcha is present and copy the request from developer tool and import it to postman and then again send the request using postman and it able to execute the request as Modelstate.IsValid returns true though the captcha is used before. Is there any way to invalidate the previously used captcha code if used again.