I originally saw this on a vanilla youtube video and searched on the bug tracker but to no avail (there aere many 'item dupe' bugs listed); sorry if this is a duplicate.

If 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.

see https://www.youtube.com/watch?v=qW0htBiMDiE about 10 mins in