Furnace duplicates xp

A furnace duplicates the XP what it previously smelt when used in a simple auto smelter system.

Steps to reproduce the bug

1. Place chest
2. Place hopper above it
3. Place Furnace above that
4. Place another hopper above that
5. Place Chest
6. Put a lever on the front of the furnace
7. Add fuel to furnace (I use coal blocks)
8. Put 32 iron ore (or a block that gives more XP or a higher quantity, this is just a realistic example) into the top chest
9. Wait for it to smelt
10. Take out the smelted items from the chest
11. Turn on the lever
12. Put something like cobblestone into the furnace
13. Take out the one stone when it smelts
14. You get a good amount of XP
15. Wait for the second stone to smelt
16. Take it out
17. You get the same good amount of XP as the last stone

Expected Result:

The second stone should give very little XP

Observed result:

The second stone gave the same amount of XP as the first stone, but the first stone gave all the XP stored. This means the XP was duplicated.

This affects all furnace types, furnaces, blast furnaces, and smokers. The top chest and hopper are not actually needed, but in a normal auto smelter they would be there. This is a very easy bug to accidentally replicate if you use an auto smelter, and can cause people to accidentally get tons of XP.