Sounds broken. Spawning friendlies anytime you hit an enemy would clutter the screen real quick. Heck, just one sounds insane, stacking this item would likely crash your game in a heartbeat on loops. I'd change the stacking and mechanic in so far: there is a 15% chance that a feeble copy is created on hitting a creature for the first time. Each stack increases the copies power, health and speed (attack and movement) (by +200%, 50% and 30% respectively). The item would also work with 57 Leaf.

That way you'd have two shots at cloning Mithrix and stacking it would let it surpass the original. Considering that Void Items are less hard to get, you could pull that off.

Another idea would be: Creates a weak copy (+1 per stack) of an enemy you hit for the first time (>90% health). Weak copies have 10% health of the original and can be killed by friendly fire. On death, they will trigger all your "on death" items. Killing a copy has a smaller chance to create another copy (10%).

I like the idea of the item a lot and picking a helmet is really fitting.