Guide on how to duplicate Armor, Masks, etc.
This works with all kinds of armor eg. Nightingdale, Dragon Priest Masks etc.

Steps
1. Buy a house with a mannequin (Hjerim or Proudspire)
2. Place the item you would like to dupe on the mannequine
3. Exit the mannequin, Then take your stuff off it
4. Drop your items that you took off your mannequin
5. Exit the house

6. Re-Enter, And your items should be on the floor as well as a duplicated version of them on the mannequin.

If you're having any troubles feel free to comment.