The duplication glitch happens when you die and quickly (between 3-7 seconds) use the windows task manager to end task (crash) "OpenJDK Platform binary" when you enter your survival world your items will be on the ground and in your Inventory.
An Interesting thing can happen when you do this glitch on a Hardcore world as you expect you will die and the death screen will appear, after doing this glitch you will cheat death (without using a totem of undying) and your items will be duplicated.