Защита от повторной отправки Post

<?php session_start(); ?>
<?php echo $_SESSION['token'] .' YY '. $_POST['token'].'<br>';


if ($_SERVER['REQUEST_METHOD'] == 'POST'
    && $_SESSION['token'] == $_POST['token']){
    echo '<script>alert("Успех");</script>';
} ?>
<?php $_SESSION['token'] = md5(time()); ?>


<form name="form" method="post" action="test.php">

    <input type="text" name="den">
    <input  type="hidden" name="token" value="<?= $_SESSION['token'] ?>">
    <input type="submit" >
<?php echo $_SESSION['token']; ?>
</form>