how do I solve this issues?
You don't unfortunately.
It quite often happens that buyers get to keep both the item and the refund.
Whatever the reason, eBay has no interest and will do nothing to support the seller. For example, eBay enables buyers to claim a refund if an item is only one day ovedue - but does absolutely nothing if the buyer than keeps the item as well, when it arrives - even with proof of delivery.
I think this is tantamount to eBay condoning buyer on seller fraud on its platform. eBay could so easily revise the user agreement to require the buyer to return either the item or the refund, but they don't. eBy simply doesn't seem to care about the seller.