Is it possible to alert a customer that they have already purchased a product that they just added into their shopping cart - or even better when they call up the description page?

I seem to be getting a number of people who think they need to repay for a downloadable product every time I update it...they don't need to, the revised download is free to people who already purchased the product.

But some people don't seem to remember this...

It would be nice to just gently remind them when that happens...otherwise I need to do a refund.