一个导出按钮事件。

点击后，弹出浏览器下载框。但是断点调试发现事件执行了2次，第一次执行完，点击“下载”，马上又开始重头执行该事件，怎么回事啊 ？？？？各位大神帮忙看看。。。

代码如下：（浏览器是搜狗的）

protected void btnExport_Click(object sender, EventArgs e)
        {
              

            DataSet ds = new DataSet();

            ds=getdata();
            if (ds.Tables[0].Rows.Count > 0)
            {
                StringWriter sw = new StringWriter();

                foreach (DataRow dr in ds.Tables[0].Rows)
                {

                    sw.WriteLine(dr["xuhao"]  );

                }

                sw.Close();

                Response.AddHeader("Content-Disposition", "attachment; filename=aa.xls");

                Response.ContentType = "application/ms-excel";

                Response.ContentEncoding = System.Text.Encoding.GetEncoding("GB2312");

                Response.Flush();
                Response.Write(sw);
               
                Response.End();
            }

}