The word ‘copy’ has nothing to do with it. Death Knell brings back all friendly Arcanysts that died prior to its summoning. Unless the code is way out there, it’s not going to make a distinction between a minion that died and one that died and was brought back. All it’s going to check for is ‘dead’ + ‘Arcanyst’. As long as it fulfills those conditions, it’ll return from the grave.
tl;dr Unless something is super weird in the code, every Arcanyst will return with Death Knell so the second Knell can summon multiple copies of the same dead Arcanyst.