Jquery投票系统，避免重复的Mysql更新

Question

I have a voting system and everything works fine except some users trying to send rapid requests in order to get around the system and add more points .

   if (isset($_COOKIE['vote_posneg_'.$id.''])){
         die("already voted");              
    }else{
        setcookie('vote_posneg_'.$id.'', 1, time()+7200);  // set cookie that expires in 2 hour "time()+7200".
    }
    switch ($section){
        case "comments":
            if($mode == "pos"){
            $sql= "UPDATE comments SET `positive`= positive+1 WHERE commentid='$id'";
            $sql2= "SELECT `positive` FROM comments WHERE commentid='$id' LIMIT 1";
            }else{
            $sql= "UPDATE comments SET `negative`= negative+1 WHERE commentid='$id'";
            $sql2= "SELECT `negative` FROM comments WHERE commentid='$id' LIMIT 1";
            }
        break;
        }

    list($cnumnow) = mysql_fetch_row(mysql_query($sql2)) or die(mysql_error());
    die($cnumnow);

Therefore before setting new cookie after updating the DB table , user can send rapid Ajax requests and adds points more than once.