You could claim that it is unprovable which authorization was first, but it is provable that the masternodes who sign more than once (regardless which one they signed first) are lying and thus must be penalized.
There is no mechanism for doing that in InstantX.
If you do create such a mechanism, its security is very difficult to correctly (in a global sense) reason about due to the large game/small game issues form Szabo's post (similar issues are raised, though somewhat glossed over, in Meni Rosenfeld's paper about Bitcoin double spending). But if really do want such a system there may be no alternative.