steps to reproduce the bug:
1.join a server
2.change your Minecraft player name
3.join the same server again
4.spawn a bot with your old name
5.kill the bot to duplicate items, and the you will be "player shadowing".

"player shadowing" means:
1.you can't move with the elytra(no trouble moving without elytra), but it's OK to destory a few blocks or place blocks.
2.you can't be seen on tab.
3.either the old name or the new name will point to the same uuid.
4.you can't use things like beds and crafting tables.
5.you can use chests and ender chests, but with no animations when it's being closed.
6.FPS drops obviously

The items you have duplicated will change as you change the items in your backpack.
Once you join the server, the bot with your old name will be kicked, and if you are not online when the bot with old name being killed, you will looked like died(your items are dropped, your experiences disappeared...) when you go online.
This is your game interface when you are "player shadowing".

emmm
I found something on bilibili.com looks like this problem:
https://www.bilibili.com/video/BV1gK411p7ek?spm_id_from=333.999.0.0
or this:
https://youtu.be/091mFU3d8m0
by Xcom6000
Maybe they're the same bug
:)