I was just messing with a build and looking for the best power to put a purpleset in, so I 5slotted Ragnarok in one power, then went to slot it in another to compare the increase, and it gave me the duplicate unique error msg as it should.
So I flipped the first power's slots to the empty alternate set, and put the 5 ragnarok in another power that took them.
Then the bug tester in me thought I'd test this; I flipped the 2nd power with ragnaroks slotted to alternate (putting the 2 ragnaroks on the same slot set; the alternate), and then hit the button to flip all slots to alternate, and it brought up both Ragnarok sets without giving the error.
It may have been done on purpose to allow for temporary slotting to find the optimal power to put a certain set in without being annoying, or it may have been an oversight the team may want to fix, idk, but I thought I'd point it out.
A purpleset is obvious, but I can see someone using this replicable method to accidentally double-slot a Numina or other unique proc and not realize.