I put my thieves guild armor on my mannequin in Solitude and now I cant keep it from duping itself. Every time I pull it off, stick it in a drawer, and put something else on the mannequin, I'll leave the building, come back, and the other gear is still in the mannequin's inventory, it's just not wearing it. Instead its wearing a brand new set of thieves guild armor. I have 10 sets of that crap now.
This is exactly what happen to me in Riften, it's not that I lose my armor it's just that I keep getting new sets. I would suggest if you guys need the armor to get the last set of armor you try this one of the houses you own. I would try it in Whiterun as getting the Riften house can take a few hours due to having to earn the Jarl's trust, and Honeyside is 8,000...9,000 if you want to the mannequins >_>.
Thanks for posting this by the way, I'm not going to sell my armor now, lol.
EDIT: I wonder if this holds true for the Dark Brotherhood...I better bite the bullet and reload my most recent save before I sold it.