There is a singleplayer duplication glitch. It involves dropping any amount of items, then pressing escape. If you keep pressing escape, eventually there will be a short period of time where the items are simultaneously in your inventory and on the ground. From here, you save and quit to title, then reopen the world, and you'll pick up the duplicated items. Please note that the glitch is only somewhat consistent. For a demonstration, please refer to this video: https://www.youtube.com/watch?v=0BWNJMp_Ri4