“The repayment or remission of import and export duties which may be made only under certain conditions and in cases specifically provided for constitutes an exception to the normal import and export procedure and consequently the provisions which provide for such repayment are to be interpreted strictly. Since a lack of ‘obvious negligence’ is an essential condition of being able to claim repayment or remission of import or export duties, it follows that that term must be interpreted in such a way that the number of cases of repayment or remission remains limited.”