DIV 的内容替换导致两次请求？

在一页面内有本站内容和他站的banner代码(放在页头)，为了防止他站页面加载缓慢不影响本站页面显示，将banner的代码放在页末加载，然后再替换到页头位置，如下
.
.
<div id="bannerdock">bann</div>
.
.
.
<div id="bannercontent">
<script>
 document.getElementById("bannerdock").innerHTML = document.getElementById("bannercontent").innerHTML;
 document.getElementById("bannercontent").innerHTML = "";
</scritp>
用httpanalyz 监控，发现其请求了二次，如下：1    09:48:00:039  0.172 s      GET     200      4074  application/x-javascript       http://ad.cn.doubleclick.net/adj/ceconline.com/Forum/HOMEPAGE;cecpo=top_leadboard;sz=728x90;ord=862959706?2    09:48:00:289  0.264 s      GET     200      1144  application/x-javascript       http://m1.cn.2mdn.net/879366/flashwrite_1_2.js                                                            3    09:48:00:289  0.015 s      GET     (Cache)  0     application/x-shockwave-flash  http://m1.cn.2mdn.net/1781494/honda728_90.swf?clickTAG=http%3A//ad.cn.doubleclick.net/click%253Bh%3Dv8/3691/3/0/%252a/e%253B195660891%253B0-0%253B1%253B25411547%253B3454-728/90%253B25434426/25452283/1%253B%253B%257Esscs%253D%253fhttp%3A//www.guangzhouhonda.com.cn/accord/index/               
4    09:48:00:585  0.031 s      GET     Aborted  0                                    http://ad.cn.doubleclick.net/adj/ceconline.com/Forum/HOMEPAGE;cecpo=top_leadboard;sz=728x90;ord=862959706?5    09:48:00:601  0.015 s      GET     Aborted  0     application/x-javascript       http://m1.cn.2mdn.net/879366/flashwrite_1_2.js                                                                                                                                                                                                                                                      
6    09:48:00:601  0.015 s      GET     (Cache)  0     application/x-shockwave-flash  http://m1.cn.2mdn.net/1781494/honda728_90.swf?clickTAG=http%3A//ad.cn.doubleclick.net/click%253Bh%3Dv8/3691/3/0/%252a/e%253B195660891%253B0-0%253B1%253B25411547%253B3454-728/90%253B25434426/25452283/1%253B%253B%257Esscs%253D%253fhttp%3A//www.guangzhouhonda.com.cn/accord/index/               
高手，这是什么原因，如何写才能保证只一次？