An old one would be fine Gonzo, just so long as its separate.
Having two instances of the same system could save you from an upgrade fail which effects are clearly visible immediately after upgrade (a delayed problem would be harder to spot and to avoid this way, though), but most probably won't save you from bugs that are triggered by something from the outside - a badly formatted flight plan message, a faulty time server, or simply an error that just happens and is likely to happen at both instances.