javascript ajax异步请求执行一次却响应了3次

xmlhttp = new XMLHttpRequest();

            xmlhttp.onreadystatechange = function () {
                if (xmlhttp.readyState=4&&xmlhttp.status == 200) {
                    alert(xmlhttp.responseText);
                    $tb.innerHTML=$tb.innerHTML+xmlhttp.responseText+"<br>";
                }
            }
            xmlhttp.open("GET",$url, true);
            xmlhttp.send();