将List += request.request.getParameter("id") + ",";
换成下面这段代码，可解决重复问题：
String id = request.getParameter("id") + ",";
//如果字符串list包含该字符串id
if (list.contains(id)) {
list = list.replace(id, ""); //将list中的id清除
}
list += id; //添加字符串id
