XHR有时会三次点击服务器

Question

我正在研究Tizen可穿戴式Web应用程序。 我正在尝试使用XHR将数据发布到服务器，

    function execute()
    var xhttp = new XMLHttpRequest();

    xhttp.onreadystatechange = function() {
        if (this.readyState == 4) {
            if (this.status == 200 || this.status == 201) {
                console.log(xhttp.responseText);

                jsonResponse = JSON.parse(xhttp.responseText);

                rheaResponse = jsonResponse.Response;

            }

        } else {
            console.log("Chat FAILED: " + this.status);
        }
    };
    xhttp.open("POST", chatUrl, true);
    xhttp.setRequestHeader("Content-Type", "application/json");
    xhttp.send(sampleData);
 }

 function onBtnTap() {
   recognizer.continuous = false;
    recognizer.onstart = function() {
        isListnining = true;
        console.log("started");
    };

    recognizer.onend = function() {
        isListnining = false
        console.log("Recogniztion API stopped");
    }
    recognizer.start();
    timer = setTimeout(stopRecognition, 8000);
    recognizer.onresult = function(event) {
        console.log("On result");
        isListnining = false;

        clearTimeout(timer);
            if (event.results.length > 0) {

            userInput = event.results[0][0].transcript;

            updateArrays(position, userInput);

            console.log(userInput);

            execute(userInput);
        }
    }

    recognizer.onerror = function(event) {
        isListnining = false
        console.log('Speech recognition error detected: ' + event.error);
    }

}

一切都运转良好，但有时我得到的反应是两次，三次以上。假设某处请求多次命中服务器。请告诉我代码中有什么问题？为什么它会多次击中服务器。