I went the barrow at labyrinthian with the wooden mask and the masks I currently had(about half of them). I entered and put on the wooden mask, transporting me to the past but since I didn't have the whole set I left and went back to my house. I always kept the masks on my aromor mannequins but since I took them with me the mannequin heads were bare when I left. Upon returning to my house I noticed that the mannequins had the masks on them yet I still had them in my inventory. I got duplicates of each mask I had except the wooden one and one other.