You can make the furnace lose its face and the furnace gets duplicated.
Steps to reproduce:
1. Recreate the setup in scr1.png by placing a furnace with a comparator facing away fryom it.
2. Place the fuel and a smeltable item in their appropriate spaces.
3. Break the furnace (if you are in survival the furnace should drop)
4. Wait for when the fuel cycle finishes.
5. The faceless furnace should have reappeared as shown in scr2.png.
6. If you were in survival, the furnace basically got duped.