[Soul-Blade]

One way would be to use a NPCs save variable, maybe assign a number to ID the type of item. When a item is placed, if checks all NPCs and if a NPC has a x/y within the radius of the potential x/y of the new item, then it checks the save, if the save is the same type as the save the current item has, then it destroys itself. Of course, another save could be used to store the data for quantity of a item, but there are a variety of ways to store that.