Asking again, what happens to packages that UPS can not deliver

Again, my friend sent a package from Las Angeles to Virginia and accidentally used a duplicate shipping label. Two packages sent, one arrived, the other UPS can not locate because of course they have it marked as delivered. What happens in this case? How can he locate and retrieve it?.

He is not too technically savvy btw. When I last asked this community for advice, a bunch of people jumped on thinking that he did this on purpose to not pay for the shipping--This is not true; he was not paying for the shipment, the recipient was and it was he who produced the labels but I think my friend printed the same one twice by accident.

Thank you for your assistance