We don't have an answer to that because nutaku has been just doing whatever the fuck they wanted. During the Phoenix and Apoc rematch, they added in an entirely new set with unusually high exchange cost, but you could get an entire max break 2nd copy of the SSRs. During Typhon and Garuda, they did not add in a 2nd set, they just left the same ones and added a few other items, so if you already had it, you couldn't get another copy.