代码执行了两次（帮忙找bug）

  var Ajax = {
    send: function(object) {
      var xhr = new XMLHttpRequest();
      xhr.open(object.method, object.url, true);
      xhr.send(object.data);
      xhr.onreadystatechange = function() {
        if (xhr.status != 200)
          object.error(xhr.responseText);
        else
          object.success(xhr.responseText);
      }
    }
  };
  window.onload = function() {
    // grab all forms with 'ajax_form' class
    var forms = document.querySelectorAll('form.ajax_form');
    for (var i = 0; i < forms.length; i++) {
      forms[i].onsubmit = function(event) { // listener
        event.preventDefault();
        var inputs = this.querySelectorAll('input.active'),
          query = '',
          form = this;
        for (var z = 0; z < inputs.length; z++)
          query += inputs[z].getAttribute('name') + '=' + inputs[z].value + '&';
        query = query.slice(0, -1);
        console.log('listener launchd');
        Ajax.send({
          method: this.getAttribute('method'),
          url: this.getAttribute('url'),
          data: query,
          success: function(responce) {
            window['form_' + form.id]['success'](responce);
          },
          error: function(responce) {
            window['form_' + form.id]['error'](responce);
          }
        });
      }
    }
  };

  form_WriteMe = {

    success: function(text) {
      alert('Спасибо за обращение!');
    },
    error: function(text) {
      console.log('error handled');
    }

  };

<form id="WriteMe" class="ajax_form" url="/ajax/letter" method="POST">
  <input name='test' class="active" value="1">
  <input type="submit" value="Отправить">
</form>