怎么判断jquery增加节点的时候内容是否重复

<!doctype html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <title>Document</title>

    <style>

        *{margin: 0; padding: 0;}

        li{list-style: none;}

        .div{width: 280px; margin: 50px auto;}

        .box_head{border: 1px solid #DDDDDD; padding: 10px;}

        .box_head li{width: 150px; height: 30px; background: #00BCF3; color: #FFFFFF; margin: 0 10px 10px 0; line-height: 30px; text-align: center; display: inline-block; cursor: pointer;}

        .box_head li.on{background: #e74303;}

        .box,.menu,.box1{margin-top: 10px; float: left; width: 260px;}

        .box li{width:200px;height: 30px; color: #FFFFFF; margin: 0 10px 10px 0; line-height: 30px; float: left; cursor: pointer;}



        .box li span.s{margin-left: 20px; color: #00BCF3; cursor: pointer;}

        .box li span.t{color: #00BCF3; cursor: pointer;}

        .box .b_a{width: 165px; height: 30px; background: #00BCF3; text-align: center; line-height: 30px; color: #FFFFFF; display: block;}

    </style>

    <script src="http://code.jquery.com/jquery-1.10.2.min.js"></script>

</head>

<body>

<div class="div">

    <div class="box_head js_box_head">

        <ul>

            <li>内容1</li>

        </ul>

    </div>

    <div class="box js_box_m">

    	<ul>

    		<li class="box_li"><input class="input_text" type="text" /><span class="s js_s">删除</span></li>

    		<li><span class="t js_t">添加input</span></li>

    	</ul>

	</div>

	<div class="box"><a class="b_a js_bc" href="javascript:;">保存到box_head</a></div>

</div>

</body>

<script>

$(function(){

  	$('.js_t').on('click',function(){

    	var html = '<li class="box_li"><input class="input_text" type="text" /><span class="s js_s">删除</span></li>';

    	$(this).parent().before(html);

    });

    $('.js_box_m').delegate('.js_s','click',function(){

    	$(this).parent().remove();

    });

  	$(".js_bc").click(function(){

        var text=$(".input_text").val();//获取标签文字

        var the_li='<li>'+ text +'</li>';//插入标签

        $(".js_box_head li").each(function fun(){//判断是否重复

            txt=$(this).text();

            if(text==txt){

                alert("该内容已添加");

                return false;

            }

        })

        if(txt!=text){//如果不重复则添加

            $(".js_box_head ul").append(the_li);

        }

         

    })

   

});

</script>

</html>