21. (1) The following income of an assessee shall be classified and computed under the head “Salaries”, namely:- (a) any salary due from an employer to the assessee in the income year, whether paid or not; (b) any salary paid or allowed to him in the income year, by or on behalf of an employer though not due or before it became due to him; and (c) any arrears of salary paid or allowed to him in the income year by or on behalf of an employer, if not charged to income-tax for any earlier income year. (2) Where any amount of salary of an assessee is once included in his total income of an income year on the basis that it had become due or that it had been paid in advance in that year, that amount shall not again be included in his income of any other year.