I have a small theory on how it happens. But it isn't solid it's just based on experience. Placing the furniture you'd have to duplicate on the ground when the server is extremely laggier than usual, It seems to duplicate that way though it isn't always assured to happen. I just know I tested and made another bookshelf from that method. But as I said this is not a solid theory. It only works like 1 out of 5 times but still.
Also like 4-Lom said which is kinda the biggest issue with my theory. You can just randomly get items into your inventory, I experienced this with a random nexus spaceship thing appearing in my inventory. Keep in mind I never purchased this item. Ever