Using /give to make a bundle that contains more items than naturally possible allows items to be duplicated by trying to put another item in the bundle. The item that you try to add to the bundle then turns into 64 or sometimes more of itself. It also means that items like tools and ender pearls can be stacked to 64.
- Give yourself a diamond (or other item)
- Right-click onto the bundle with the diamond
→ The diamond's item count increases