重新加载页面值时自动添加

Question

我单击了提交，并将值添加到了db中，但是当重新加载页面值时，如果我又添加了一次，  再次单击，值将添加到db

<?php
if(isset($_POST['add_new'])){
    if($_SERVER['REQUEST_METHOD'] == 'POST'){
        $usName = $_POST['usName'];
        $date   = $_POST['date'];
        $email  = $_POST['email'];
        $status = $_POST['status'];

        $errors=array();
        if(!empty($usName) and !empty($date) and !empty($email) ){
            $stm1= $con->prepare("INSERT INTO members(username,date,email,status) 
                                  VALUES (:usN,:Date,:Mail,:status)");
            $stm1->execute(array(
                ":usN"=>$usName , 
                ":Date"=>$date , 
                ":Mail"=>$email , 
                ":status"=>$status)
            );
            $success="the member has added";       
        } else {
            $errors[]='all fildes are required';
        }
    } else {
        echo 'bad';
    }

}
?>