asp.net mvc利用Json验证数据和导向页面，解决重复提交问题

//HTML代码
   <script src="http://www.cnblogs.com/Repository/JavaScript/Scripts/jquery-1.4.1.min.js" type="text/javascript"></script>
    <table cellspacing="0" cellpadding="0" width="100%" border="0" height="143" id="table212">
                                                        <tr>
                                                            <td width="13%" height="38">
                                                                <span class="login_txt">用户名：</span>
                                                            </td>
                                                            <td height="38" colspan="2">
                                                                <input type="text" name="username" id="username" maxlength="15" class="textStyle" />
                                                            </td>
                                                        </tr>
                                                        <tr>
                                                            <td width="13%" height="35">
                                                                <span class="login_txt">密&nbsp;&nbsp;&nbsp;&nbsp;码：</span>
                                                            </td>
                                                            <td height="35" colspan="2">
                                                                <input type="password" maxlength="15" id="password" name="password" class="textStyle" />
                                                            </td>
                                                        </tr>
                                                        <tr>
                                                            <td>
                                                                &nbsp;
                                                            </td>
                                                            <td style="width: 155px; font-weight: bold; color: Red; font-size: 12px">
                                                                <div id="msg">
                                                                </div>
                                                            </td>
                                                            <td align="left">
                                                                <input id="submitbtn" name="submitbtn" type="button" class="jhp" value="&nbsp;登 录&nbsp;"
                                                                    title="登录" />
                                                                <script type="text/javascript">
                                                                    $(function () {
                                                                        $("#submitbtn").click(function () {
                                                                            var username = document.getElementById("username").value;
                                                                            var userpwd = document.getElementById("password").value;
                                                                            var msg = document.getElementById("msg");
                                                                            msg.innerHTML = "";
                                                                            if (!username) {
                                                                                msg.innerHTML = "请输入用户名";
                                                                                return false;
                                                                            }

                                                                            if (!userpwd) {
                                                                                msg.innerHTML = "请输入密码";
                                                                                return false;
                                                                            }
                                                                            var url = '../Login/LoginVar.aspx?loginid=' + username + '&loginpwd=' + userpwd;
                                                                            $.get(url, { date: new Date().getTime() }, function (data) {
                                                                                if (data) {
                                                                                    if (data.url) {
                                                                                        window.location = data.url;
                                                                                    }
                                                                                    else {
                                                                                        msg.innerHTML = data;
                                                                                    }
                                                                                }
                                                                            }, "json");
                                                                        });
                                                                    });

                                                                </script>
                                                            </td>
                                                        </tr>
                                                    </table>

　　

// 后台Action代码

  /// <summary>
        /// 登陆验证
        /// </summary>
        /// <param name="fc"></param> 
        public ActionResult LoginVar(string username,string userpwd)
        {            
            try
            {
                ZHUser user = userRepository.GetUser_username_password(username, Tools.GetMd5By16(userpwd));
                if (user == null)
                {
                    var json = "账号或密码不正确";
                    return Json(json, JsonRequestBehavior.AllowGet);
                }
                else
                {
                    FormsAuthentication.SetAuthCookie(username, false);
                    HttpCookie userCookie = new HttpCookie("username", user.UserName);
                    HttpCookie userid = new HttpCookie("userid", user.Id.ToString());
                    Response.Cookies.Add(userid);
                    Response.Cookies.Add(userCookie);
                    var json = new { url = Url.Action("Index", "Main") };
                    return Json(json, JsonRequestBehavior.AllowGet);
                    //return RedirectToAction("Index", "Main");   
                }
            }
            catch (Exception ex)
            {
                var json = ex.Message;
                return Json(json, JsonRequestBehavior.AllowGet);
            }
        }