When making a payment using paypal, the first thing you do is to contact the person that the payment was made to. If they don't reply, then open a dispute ticket with PayPal. PayPal will also contact the vendor to get their statement. You tell PayPal that you accidentally made a double payment for the same product and you are asking the vendor to issue you a refund but they are not answering you. If a vendor gets enough complaints from buyers, PayPal can remove that vendor from their system. If you paid using a credit card, you can also file a dispute with your credit card provider. I don't advise using debit cards to pay for any modules in the marketplace.

On UNA side of things, any vendor that does not handle customer issues should be removed from the marketplace; at least disabled until the issues have been properly handled.