It is possible to duplicate any item or block using a furnace or a dispenser. Although it's not 100% reproducible, so it might take several tries. It also seems to work much better with dispensers compared to furnaces.
Steps to Reproduce:
1. place a furnace or dispenser
2. begin to break the furnace with a pickaxe, right before the furnace breaks switch to a different tool, like a sword or shovel, continue breaking the furnace/dispenser. The furnace/dispenser should disappear for about a half second, and then reappear.
3. place the item you want to dupe inside the furnace/dispenser, rapidly left or right click on the item, keep clicking as fast as you can. Eventually the furnace/dispenser will break. the furnace/dispenser will disappear and not drop an item, but the item inside will occasionally drop two of it self.
4. You should have 2 of the same item now, to pick up the duped version, drop the item out of inventory, you will pick up the duplication item, duplicated blocks can be placed infinitely, and items (eggs, ender pearls, etc) will never run out
this video also shows the actual process: http://youtu.be/C7iaAypwBDw