Reeeeally? The manaquin left of the door duplicates? Is this upon every entry? Do you have to try putting something else on the manaquin before it will start duping? I ask because i would like multiple copies of telvanni robes, and maybe even miraaks robes for mage followers. a little telvanni hord if you will.
This is what happened:
I put my Elven armor on it, and then switched to a full suit of cultist robes. No prob.
Then I acquired some Stalhirm armor and removed the cultist gear and put the Stalhrim on it. When I returned to the house, the armor wasn't equipped (but was still in the mannequin's inventory), and the mannequin had generated new cultist robes, gloves and boots I removed them and re-equipped the Stalhirm.
Returned later, same thing. So I removed everything off the mannequin.
Returned later--it had generated another set of robes, gloves and boots.
So I tried putting different things on it--Thalmor robes, adept robes, enchanted ebony armor, whatever else I had. Each time when I would return (or went to sleep and woke up) it would de-equip what I put on there and generate new cultist robes, gloves and boots.
The amusement with this has worn thin so now I'm resigned to letting the mannequin win and keep its cultist get-up. As long as I leave the cultist robes, gloves and boots on there, it doesn't generate more. It's only when I remove the ones that are on there.
I have no idea if this glitch would generate anything else. But I did read somewhere that some people have had that same mannequin de-enchant enchanted items placed on it (which is why I have not put my Deathbrand armor on any of the mannequins!). I haven't had that happen so far, thankfully.
But I suppose, if I ever decide to start my own cult in Solstheim, I have an easy, no-cost way of outfitting my minions!