From and after the commencement of this Act, where under any feu it is provided that on the entry of an heir or singular successor, or on any specified periodical occasion, payment shall be made of a duplicand or double of the feu-duty, or that the feu-duty shall be doubled, such payment, whether or not it is provided to be made at the same term or terms as the feu-duty, shall be deemed to be inclusive of the feu-duty for the year, and the amount of the casualty payable to the superior shall, for the purposes of the Feudal Casualties (Scotland) Act, 1914, and for all other purposes, be deemed to be the amount of one year’s feu-duty only, unless such payment is expressed to be over and above such feu-duty or is otherwise unequivocally declared not to include the feu-duty of the year.