Glitch Status: Unpatched
[*]Completed Dark Brotherhood questline
[*]Have achieved the Mannequin upgrade in the Sanctuary.
[*]Travel to Dawnstar (location of Sanctuary, but you already knew that...).
Pretty simple. All you need to do is activate the mannequin and store the apparel you need to dupe inside; however, you must not exit out of the deposit menu. After you deposit - and without exiting the menu - withdraw your items. Equip your apparel, and exit the sanctuary. Upon re-entry, you should find that the mannequin is wearing your previously deposited armor! The nerve... Oh, wait... YOU ARE WEARING IT TO! What the deuce? Whatevs, you now have two sets. Continue as necessary and enjoy.
1. Do above steps.
2. ???? (Just kidding, it's really fucking obvious)