Two User Logon Not Allowed on same Machine

create or replace trigger TG_SINGLELOGIN
after logon on database
declare
cnt pls_integer;
begin
select count(*) into cnt from v$session
where username !=sys_context('userenv','session_user')  and terminal =sys_context('userenv','terminal');
if cnt >= 1 then
raise_application_error (-20001, 'You are already connected on another user with this machine');
end if;
exception
when no_data_found then raise;
end;
/