Tigger that prevents players from picking up more than one of the same item?

How do you make a trigger that won't let a hero pick up more than 1 of the same item?

EDIT:
Doh i posted in the wrong section, and i cant seem to figure out how to delete it. Im sure someone will move it for me as always.

Event - Unit Acquires item
Condition -
Or
Triggering unit has item of triggering item in slot1
Triggering unit has item of triggering item in slot2
Triggering unit has item of triggering item in slot3
...
Action
Drop item from triggering unit

Yes... Im having a similar problem and i dont think that trigger will help. In my map i have made some mods to allow hero's to gain the weapon graphic in their hand when they pick up a weapon (Its really kool, im thinking of doing a tourturial on it) and i want to make it so that some classes (warlocks) cant pick up Dual Swords etc..... I also dont want hero's having 2 weapons.. any good ways how?

Put all weapons in an item class, like permanet, then do a trigger like this:
Events - A unit is issued an order targeting an item ( i don't remember if this is the right event )
Condition - Item type of Target item of issued order equal to Permanent
Item class of item carried on slot 1 of ordered unit is equal to permanent(do this for the 6 slots)
Action - Unit - order ordered unit to stop
You can put an error message here using Game - Dysplay text message.

Why detect order? A hero can also be ordered to attack an item. Just use the pickup item event.

This would instantly make them drop the item upon aquiering it.

that's why conditions exist.

and by the way:
http://www.wc3campaigns.com/showthread.php?t=42798