ajax聊天页面提交两次，PHP/Jquery/Mysql

我有一个非常基本的聊天页面。chat.php基本上有一个文本区域，用户键入一些东西，点击回车，然后提交表单，ajax表单将chat_window.php加载到#refreshdiv中。第一次提交后，第二次提交由于某种原因是提交两次，然后第三次提交3x。。等等。我还有一个间隔，每隔5秒用chat_window.php重新加载一个div。

chat.php

<div class="container">
<?PHP

 echo "<form id='chat_form' method='post' action='processforms/process_chat.php'>";
 echo "<input type='hidden' name='chat_form_submit'>";
 echo "<textarea name='msg' style='width:98%;padding:4px;' id='msg_field'></textarea>";
 echo "</form>";
 echo "<div id='refresh'></div>"; //REFRESH WINDOW (chat_window.php)

?>
</div><!-- END CONTAINER -->

chat_window.php

<?PHP 
 include ("../includes/mysql.php"); 
 $q = mysql_query("SELECT * FROM chat ORDER BY timestamp DESC");
 while ($row = mysql_fetch_assoc($q)) {
  echo $row['msg']."<br>";
 }
?>

Javascript

<script src="js/jquery.js" type="text/javascript"></script>
    <script src="js/jquery.form.js" type="text/javascript"></script>
    <script type="text/javascript">
    $(document).ready(function() {// REFRESH INTERVAL
      interval = setTimeout(refreshpage, 5000);
      function refreshpage() {
         $('#refresh').load('includes/chat_window.php?timer='+new Date().getTime(), function () {
          interval = setTimeout(refreshpage, 5000);
         }); 
      }    
    });
    </script>
    <script type="text/javascript"> //CHAT FORM
    $(document).ready(function() { 
        var options = { 
            target:        '',
            dataType:      'html',          
            beforeSubmit:  showRequest_chatform,        
            success:       showResponse_chatform
        }; 
        $(".container").delegate("#chat_form","submit",function () { 
            $(this).ajaxSubmit(options); //submit the form
            return false;
        });
    });
    function showRequest_chatform(formData, jqForm, options) {
        return true; 
    }
    function showResponse_chatform(responseText, statusText, xhr, $form){   
         $('#refresh').load('includes/chat_window.php', function () { 
           $('#msg_field').val("").focus();
         });
    }
    </script>