Whenever a customer tries to buy a product that they have already purchased in the past, AND the product's "Automatic redelivery" flag is set to "allowed", instead of charging the customer, simply redeliver the product instead with a message to the customer.

This should also apply to those products purchased as a gift:

1) if the original purchaser tries to buy the same product to the same gift recipient: redeliver with a message.

2) if the gift recipient tries to buy the same product they received as a gift: redeliver with a message.

3) if someone else tries to buy the product as a gift and the recipient already has it: redeliver with a message.

Yes, there is a nice big banner on the product page about already purchased the item, but sometimes people get distracted, or the product was in cart already, and duplicate orders happen and creates extra work for the merchant.

The Caspervend product (and I'm sure other vending products do as well) have a duplicate order check that auto-refunds the customer if they try to buy a product they have already purchased in the past through that system. The Marketplace should have similar safeguards.