If you take an item (or stack of items) out of a container right as it's removed by redstone, and the server thread is under heavy load, then the client and server can disagree on whether the stack has been removed or not. This means that the client can sometimes take the same stack out twice, making the appearance of item duplication.
While the duplicated items disappear when the container's GUI is closed, they persist for as long as it's open. I've reproduced this with hoppers and droppers in singleplayer, but so far I haven't tested any other containers, or tested the bug in multiplayer.
Comments 5
It's my singleplayer creative world, and it's on vanilla with one datapack that just lets me put TNT into beehives.
I took a video demonstrating it, but I couldn't get it small enough to upload, so I put it on YouTube instead - https://youtu.be/n6PDPlOHIG0
Please attach a screenshot of this occurring while the F3 debug screen is enabled.
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Does the server use any modified software like Paper or Spigot?