[Trigger] Need help with a item trigger

Level 4
May 27, 2012
Well, I'm quite new to triggers, and I need some help on making a trigger that does the following:

Makes it so when somebody buys a item from the artifact shop, they can't buy the same artifact twice or cant hold two of the same artifact, but can buy other artifacts.
Artifact being items in an artifact shop.

Thanks in advanced.

PS: I'm currently working on a Enfo's 3way map, and its turning out pretty amazing so far, if you'd like to help, just send me a message.