Spiked bat can be macroed with leftclick on mobs to get far more drops like endstone dust from enderman.
The actual dupe bug is related to crafting anvils and hammercrafting, put GT double plate plate recipe x2 inside and craft them fast by manualy clicking fast or with macro. That may sometimes result in extra crafted item. Allows duping any type of metal that can be made into rods/double rods. I'll provide gyazo GIF image if my explanation is unclear.
The bat thing: https://gyazo.com/01e60349418601ecfcac611d54e55bbc
The anvil thing: https://gyazo.com/4cf40120091ecd50f8d64087b5837c1a