如何限制同一用户名同时登陆

Function CheckOnline()	DIM IP,rsPrv,Sql,PrvDbState	PrvDbState = False	If DBSTATE = False Then		DbOpen()		PrvDbState = True	End If	Set rsPrv=Server.CreateObject("ADODB.Recordset")	If Session("UserName") = "" then		Sql="select * from [Online] where SessionID='"& Session.Sessionid &"'"		rsPrv.Open Sql,Conn,1,3		If rsPrv.Eof then			rsPrv.AddNew			rsPrv("SessionID") = Session.SessionID			rsPrv("GroupChargeLv") = -1			rsPrv("LastActTime") = Now()			rsPrv("UserIP") = GetIP			rsPrv("OnLineTime") = 0			rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")		Else			rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")			rsPrv("OnLineTime") = rsPrv("OnLineTime") + DateDiff("n",rsPrv("LastActTime"),Now())			rsPrv("LastActTime") = Now()		End If		rsPrv.Update		rsPrv.Close()		'response.Write "notlogin"	Else		'response.Write "logined"		Sql="select * from [Online] where UserName='"& Session("UserName") &"'"		rsPrv.Open sql,Conn,1,3		If rsPrv.Eof then			rsPrv.AddNew			rsPrv("SessionID") = Session.SessionID			rsPrv("UserName") = Session("UserName")			rsPrv("GroupChargeLv") = Session("GroupChargeLv")			rsPrv("LastActTime") = Now()			rsPrv("OnLineTime") = 0			rsPrv("UserIP")= GetIP			rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")		Else			If rsPrv("SessionID") <> Session.SessionID And Application("LoginSet")(1) = False Then				InfoTo "LoginOut.asp","该帐户已在其他地方登陆，网站设置1个ID只能有1个登陆/n你可以稍候尝试登陆。"				Response.End()			End If			rsPrv("UserWhere") = Request.ServerVariables("HTTP_REFERER")			rsPrv("OnLineTime") = rsPrv("OnLineTime") + DateDiff("n",rsPrv("LastActTime"),Now())			rsPrv("LastActTime")=Now() 		End If		rsPrv.Update		rsPrv.Close()	End If	Set rsPrv = Nothing		If DateDiff("s",Application("OnLineLastDelete"),Now()) > Int(Application("DELETEONLINEDIFF")) Then		Application.Lock()			Application("OnLineLastDelete") = now		Application.UnLock()		Conn.ExeCute("delete from [Online] where datedIff('s',LastActTime,Now())>"&	Int(Application("CHECKONLINEDIFF") &"")) '删除x秒没有活动的访客	End If		If PrvDbState = True Then DbClose()	End Function