already has that particular item in their shopping cart.

Greetings,
I require the above functionality because I sell unique items, and while oscommerce will display an error message if the customer orders an item that doesn't have sufficient quantity, I want to avoid having the customer go through these extra steps.
So if a customer has added an item to their shopping cart, and they go back to the item from within the catalog, is there a way to disable the "add to cart" button?

thanks for your help.
pulp