[2020] possible item dupe bug

I hadnt played osrs for a few months (well before the POH storage update). I only had camo stored in there. I got mems last night, and just received my first frog token. I checked the fancy dress box and it only had camo stored. I teled to varrock, exchanged for prince outfit, and went back to POH. The fancy dress box now has prince outfit in it, as well as my copy in the inventory. I cant store a duplicate set it says, and its the same in vanilla. possible bug?