Dupe

you drop an item/item stack on the ground and leave the game in the exact tick you pick the item up, when you relog the item/item stack will be both in your inventory AND on the ground, and you will pick the stack on the ground up in the next tick, resulting in the item being duplicated.

YouTube Video : https://www.youtube.com/watch?v=MD5D90pY1uA

About 4:10 in

Thanks - Alum