It shouldn't overwrite though, unless the nocturnal shield would have given a higher value damage shield compared to what was already on (including if it has already taken some hits, thus having a lower absorption threshold).
Edit : After going out of my way to test this, nocturnal will overwrite galv regardless of the latter's durability compared to the former's potency. You have to give us the benefit of the doubt though, no one parties with AST in end game raid to see this sort of thing ;A;
