It happens and we have all been there and done that. But, you, as a merchant, need to solve this. How?
Again, this is best handled when looking at it case-by-case. If this happens rarely, you can just go ahead and process the return. And again, please keep in mind that customers might file a chargeback if they returned the items and did not receive a refund.
When this happens often though, please make sure to communicate to your customers your return policy upfront, either through the Approved Email Template or your Returns Policy page on your website.