CREATE TRIGGER E_LOGON_LIMIT_SA
ON ALL SERVER 
FOR LOGON
AS
BEGIN
IF EXISTS(SELECT 1
          FROM   sys.dm_exec_sessions
          WHERE  is_user_process = 1 
            AND   ORIGINAL_LOGIN()= 'sa'
            AND    original_login_name = 'sa'
          HAVING COUNT(*) > 1)
    ROLLBACK;
END;
