This isn't a huge glitch but, if you give someone like one good item then can switch to creative. put a item frame on the wall. put item in it. then break the frame making 2 of the same item.
P.S. i know they already have creative but with good stuff you might not want them to beable to duplicate it. Ex. high level enchants.
I'm going to go out on a limb and resolve as working as intended. If they have creative, anything is fair game. They could easily get bottles of enchanting and enchant a bunch of items.