You can duplicate any item by a very easy method. Simply find the 
item you want, then save. Pick the item up from the ground or your
inventory so it replaces your cursor, but do not drop it or put it
in your inventory, Reload your save file. The item should still be
where you picked it up from AND on your cursor. 
You can do this to duplicate rare, powerful items like Mind Crystals,
Knowledge Brew, and one of a kind weapons. Try selling duplicate 
Demon Slayers to make ridiculous amounts of gold lightning quick.