I cannot think of a reason why a customer that already bought a copiable item would want to pay for it again, and as a merchant, refunding accidental double purchases is a manual process (where we also "eat" the MP seller fee). I would suggest a setting for stores to not allow double purchase and instead give the customer a button to redeliver. This is an option in Caspervend which prevents a lot of mistaken double purchases and makes customers lives easier.
For extra bonus points, if we ever get tighter integration of Caspervend and MP, it would be amazing if "duplicates" were universal for both systems. For example, today a customer buying something a second time in my store using CV - even if the first purchase was on the MP! - will get a message, refund, and redelivery. But the reverse is not true, the MP will happily take their money again for the same product. It would be nice for this to be synced in both directions.