I built the Armory addition at Lakeview manor, and gave one of the armor mannequins some vampire armor from Dawnguard. Later, I changed my mind and took its vampire armor away (removed it from its inventory) and put a set of wolf armor on it.
Now, whenever I leave the house, the next time I come back it is wearing vampire armor on its torso. I take the vampire armor away, leave the house, come back, it's made another one. One time I took away everything it had and left, and when I came back it had duplicated every item; the vampire armor, the wolf armor, even the ring of hircine.

I had the same problem with two different mannequins.

I fixed it by opening the console. Clicking the mannequin. Typing drop For example "drop 000cae15 1". The item should drop to the feet of the mannequin. Done.

It's a well known bug I believe. I know some people use it as a get rich quick function.