App to stop customer from purchasing product twice.

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I sell digital download files. Once purchased the customer will never need to purchase the same product again. I am looking for a way for the customer to easily know if they have purchased a particular file in the past without having to go back and forth to their order history. Maybe an alert if they add a previously purchased design to the cart, or a check mark on the image? Just something to help eliminate duplicate purchases. Is there a way to do this within Shopify or a Shopify app? 

I appreciate any advice given!