When shift-clicking items into the crafting table while previewing crafting recipes causes both items to be shown at the same time. This is easily replicable.

1. Press a crafting recipe in the recipe book that you do not have the necessary items to craft.
2. Shift-click any item into the crafting table.
3. You will notice that both textures are overlayed on top of each other, and hovering on the double item also shows the name and description for both at the same time.

Video proof is attached.