判断页面是否引用了jquery避免重复

<script>!window.jQuery && document.write("<script src=\"/js/jquery-1.4a2.min.js\">" + "</scr" + "ipt>");</script>

 

<script type="text/javascript">
window.jQuery || document.write("<script src='~/Content/assets/js/jquery/jquery-2.0.3.min.js'>" + "<" + "script>");
</script>