为什么我试图通过ip地址检查双重投票，却不能正常工作？

if ($qtype === "sc"){
                // single choice has only one select item

                // check whether user has already answered this question
                $i=0;
                $res = mysqli_query($mysqli, "SELECT ip FROM votedanswers WHERE ip = \"".$ip."\" and qID = \"".$qID."\"");

                while($obj = mysqli_fetch_object($res))
                {
                  $i = $i+1;
                }                   
                if ($i > 0){
                    echo "your vote has already been counted";
                }
                else{
                    //insert data
                    $res = mysqli_query($mysqli, "INSERT INTO votedanswers (ip, qID, qQuestion, aID) VALUES ('$ip', '$qID', '$quest', '$aID')");
                    echo "your vote has been counted";
                }   

        } else {
            echo "nope!";
        }`

不久前测试过，一切正常。但现在我想投多少就投多少。