If the guarantor has paid without notifying the debtor, and the latter not being aware of the payment, repeats the payment, the former has no remedy whatever against the debtor, but only against the creditor. Nevertheless, in case of a gratuitous guaranty, if the guarantor was prevented by a fortuitous event from advising the debtor of the payment, and the creditor becomes insolvent, the debtor shall reimburse the guarantor for the amount paid. (1842a)