I've noticed when a customer adds a product to their cart then edits the order, it doesn't edit the item in their cart but instead addes a new identical item to the cart with the new option selected.

Lets use a shirt offered in S,M,L sizes as an example.
1. Customer goes to Shirt page selects Size M and adds to cart
2. Customer decides they really want a Small and clicks on the item in the cart to modify the order.
3. Zen-cart pulls up the item from the cart with the Shirt showing the Medium size the customer originally selected.
4. Customer changes from M to S and clicks the "Add to Cart" button

Now the shopping cart shows 2 shirts in the cart 1 in Medium and the other in Small.

Any suggestions on how I might go about changing the function of the "Add to Cart" button into more of a "Save Changes to Item" button? Where it modifies the original item clicked on rather than adding a brand new item to the cart?

Thanks!