Well, I have the same problem, I would like some piece of code that I can add to my "add to cart" link so that the cart automatically reset the number of items present in the cart so that buyer can only buy what he clicked last. Is that possible?
I only sell one item in different spots of my website, just like the person who started this thread, so I don't want that buyers keep clicking and have the same item multiple times in their chart.
I prefer to use the shopping cart rather than buy now button, because it's much easier to connect the Google E-Commerce Stats.
PLEASE let's find a way!