If downloads unlimited, instead of warning about purchasing files, disallow (with message saying they have file already)

When a limited number of download attempts is used, it may make sense (to some people) to allow a user to purchase additional download chances (with a warning).

But if a user has already purchased a file, and download limit is set to infinite (which I think is most fair), they should not be allowed to accidentally re-purchase a file. They'd be getting nothing for their money, which is unfair, and could create legal and ethical problems for the site owner and customer.

If a file download limit is set to infinite, when a customer attempts to accidentally re-purchase the file product, they should either be told that they already have the product and can download it whenever they like, or should be charged $0 for it - something that keeps them from spending money for nothing.

This would also reduce the number of superfluous refund and customer service interactions that would necessarily result when someone accidentally pays for the same non-physical item twice.