Hi! I was working with a datapack for 1.17 (the one I uploaded here) to resolve a bug of the datapack. I found a minecraft bug. It seems /item could duplicate items and create strange ghost items. As you can see in the screenshots, if I check for the existence of the backpack item the game returns true, and if I ask the nbt it gives me the nbt of the item. However, that item should be there, but not 'ghost' and the other backpack(in my inventory) should exist. In this way, the command duplicates(not always) the items.
Do you want to check? Try this datapack, maybe with resource pack (search on the web for it, Backpacks Datapack). Give you a backpack with /loot give @s loot ulg:backpack/backpack or craft it. Place it in the off hand, right click, go to your inventory. Take the opened backpack to a slot of the inventory(not the hotbar slots). It will return on your second hand. Try again and again putting it in the inventory. At a moment, it will not return to the second hand. Now try checking for the item using /data get or predicates, or if data o [nbt={}] arg. You'll find it.
Take an item in your hotbar, select it and press F or the switch item button you have.
You will have successfully duplicated the backpack.

I don't know how to explain it, but at least I can post this try datapack and make you check and study the problem. For a better and less-laggy minecraft. Thanks

Steps to Reproduce:

1. Ensure you are in Creative mode with a clear inventory
2. Stand on the ground and drop a block of stone (item form) directly downwards
3. Open the Survival tab of the Creative inventory
4. Wait until you pick up the stone
5. Without closing your inventory, move the stone to a different hotbar or inventory slot
6. Close your inventory
7. Right-click on the ground while the first hotbar slot is the active (main hand) slot
   → A block of stone is placed, even though your hand is empty