如何防止同一帐户重复登录系统

如何防止控制客户端使其用同一帐户重复登录系统.
解决思路：

维护一online表，查看有登陆，就不允许再次登陆，以sessionid作为唯一标识符号，也可以产生一个ＧＵＩＤ发到ＣＯＯＫＩＥ中，以区分不同的ＣＬＩＥＮＴ，再佐以ＪＳ，可以达到更好的效果，比如离开后自动离线

解决代码：

public virtual void Application_Start(object sender, EventArgs e)
{
// reset the mailer indicator
Application["MailerStatus"] = "All Mailings Complete";

// initialize a datatable for users online
DataTable objUserTable = new DataTable();
objUserTable.Columns.Add("SessionID",System.Type.GetType("System.Guid"));
objUserTable.Columns.Add("PeopleID",System.Type.GetType("System.Int32"));
objUserTable.Columns.Add("ShowDetail",System.Type.GetType("System.Boolean"));
DataColumn[] pk = new DataColumn[1];
pk[0] = objUserTable.Columns[0];
objUserTable.PrimaryKey = pk;
Application["UserTable"] = objUserTable;
}

/**////
/// The Session_Start event adds user session information to
/// Application["UserTable"].
///
public virtual void Session_Start(object sender, EventArgs e)
{
Application.Lock();
//Application.Lock ();
DataTable objUserTable = (DataTable)Application["UserTable"];
DataRow objRow = objUserTable.NewRow();
Guid objGuid = Guid.NewGuid();
objRow[0] = objGuid;
Session["PfSessionID"] = objRow[0];
objRow[1] = 0;
objRow[2] = false;
objUserTable.Rows.Add(objRow);
Application["UserTable"] = objUserTable;
Application.UnLock();
}

/**////
/// The Session_End event deletes user session information from
/// Application["UserTable"].
///
public virtual void Session_End(object sender, EventArgs e)
{
Application.Lock();
DataTable objUserTable = (DataTable)Application["UserTable"];
objUserTable.Rows.Find((Guid)Session["PfSessionID"]).Delete();
Application["UserTable"] = objUserTable;
Application.UnLock();
}