Furnace XP duplication

If you smelt a single item in the furnace, you can swap the smelted item with a stack of the same item repeatedly. Each time you swap the item or stacked item, you're given the XP bonus for that smelted item. This allows you to duplicate the XP for the item repeatedly earning an insane amount of XP.

Steps to reproduce:

1. Get a stack of gold ingots (can be any smeltable item, however gold is very easily attainable and gives a lot of xp)
2. Smelt one raw gold in the furnace
3. Take the stack of gold ingots and replace the one gold ingot in the furnace output slot with the stack and then immediately replace the stack with the one gold ingot you just pulled out. (This can be done repeatedly - simply by double-clicking the output with the stack of gold ingots selected).

The result will be an xp gain for the single gold ingot, followed by an xp gain for a stack of gold ingots.. and the furnace will continue this repeatedly as long as you do not remove all items from the output slot.