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.
[media]3. Re-enter the world and you will have the inventory of the account you were originally on.
[media]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.
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-697, which is a report that has not been resolved yet.
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