In a single-player world (tested on 1.16.1, might affect other versions), inventories can be duplicated by changing your account in the Minecraft launcher, then opening the world.

Steps to recreate the issue:
1. Open a world with items in your inventory.

2. Exit the world and switch your account in the launcher.

3. Re-enter the world and you will have the inventory of the account you were originally on.

These items are not ghost items. They can be put into a chest, and you can return to your original account and still have the items in your inventory, thus duplicating the items.

So far, I am aware that player inventories and ender chests are duplicated. I am not sure if anything else is.