In survival, first place down a armor stand. Then, select an armor piece (in my case, a diamond chestplate) and repetitively press Attack and Use at the same time. Sometimes, this will actually steal your items.
Link to video: http://youtu.be/OYCSTcCsinE (The video may still be uploading, give it a minute)
confirmed.
though i noticed something very weird with the duplicated item. it works just like normal, but CANNOT be shift+clicked at anytime. furthermore; if you right click to equip the duplicated armor piece, it dissapears.