You can duplicate tools and armor items in the crafting grid when the gamerule doLimitedCrafting is set to true.
- Activate limited crafting
- Switch to Survival mode
- Get 2 pieces of the same armor or tool (it do not matter with the durability left, it can even have full durability)
- Go in a crafting table or just open your inventory
- Put both items in the grafting grid
- Press E to close the GUI
→ Both items will be moved back to your inventory but additionally one will be dropped on the ground if you are lucky. This item is not a ghost item and can be picked up. Its about 50/50 if you get an extra one.
→ If you have 2 different items, it seems to be the one you didn`t put first in the crafting grid. Example: I have 2 diamond swords, one I never used, and 1 I have used a lot. If I place the one i never used first into the crafting grid and I am lucky to get an extra sword that sword will then be the used one, because I placed the unused one first in the crafting grid.