一个账号不能同时登陆

if (HttpContext.Current.Cache[guid.ToString()] == null)

{

logger.Info(string.Format("用户:{0},IP:{1},机器名：{2}成功登录", txtUserName.Value,Request.UserHostAddress,Request.UserHostName));

FormsAuthentication.RedirectFromLoginPage(guid.ToString(), chk.Checked);

}

else

{

logger.Info(string.Format("用户:{0}，IP:{1},机器名：{2} 验证成功,但已有人使用同用户名登录", txtUserName.Value, Request.UserHostAddress, Request.UserHostName));

this.RegisterStartupExtAlertDialog("login_having", "已有人使用相同用户名登录，请确认你的账户没有被盗用！<br/>如果是因为你上次意外退出，请稍等20秒钟后重试！");

}

<asp:Timer ID="Timer1" runat="server" Interval="10000" OnTick="Timer1_Tick" Enabled="false" />

protected void Timer1_Tick(object sender, EventArgs e)

{

HttpContext.Current.Cache.Insert(this.UserID.ToString(), DateTime.Now, null, DateTime.Now.AddSeconds(20), TimeSpan.Zero);

}