21However, by way of exception to the principle of the reimbursement of charges incompatible with EU law, the repayment of duties wrongly levied can be refused only where repayment would entail unjust enrichment of the persons concerned, that is to say, where it is established that the person required to pay such charges has actually passed them on to the purchaser directly (see, to that effect, Lady & Kid and Others, paragraphs 18 and 20).