Who knew bumping was legit?
Anyways, you just have to do this, with a variable
Unit acquires an item.
Conditions
ItemCheck=True
Actions
Set Itemcheck=False
Drop item being manipulated
If Hero has item of type of item being manipulated
Then- Give text message: ONLY 1 ITEM ALLOWED
Else- Give Item being manipulated to triggering unit.
Set ItemCheck=True
For the items, make it fire when they die, and a chance. Then give them a random level X item. Just set all desired items to level X