Equipment can be duplicated if you have an item equipped, at least two more of the same item in your inventory, and you try to replace the equipped item with the stack of items.

Ex: You have a sword equipped. You have a stack of 2 swords in your inventory. Click on the stack of 2 swords and then click on the equipped sword. You'll now have 1 sword equipped and a stack of 3 swords. Every time you click, your swords will double and  -1. So if you keep clicking, you'll get 5, 9, 17, 33, 65, 129, etc. swords.