easyui 中layout奇怪有关问题 _Web前端

easyui 中layout奇怪问题
请问以下alert为何被执行两次

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<!--引入页面所需的样式文件-->
<link rel="stylesheet" type="text/css" href="jquery-easyui-1.2.4/themes/default/easyui.css">

<!--引入页面所需的脚本文件-->
<script type="text/javascript" src="jquery-easyui-1.2.4/jquery-1.6.min.js"></script>
<script type="text/javascript" src="jquery-easyui-1.2.4/jquery.easyui.min.js"></script>

<style type="text/css">   
</style> 
</head>
<body>
<div id="cc" class="easyui-layout" fit="true">  
    <div region="north" style="height:100px;"></div>   
    <div region="center">
		<script type="text/javascript">
			alert('11');
		</script>
	</div>  
</div>  
</body>
</html>