§ 1231. Certain restrictions against double taxation. Except as expressly otherwise provided in this article, nothing in this article shall limit or prevent the imposition of a tax on gross income or a tax on gross receipts of persons, firms and corporations doing business in any city with a population of one million or more. No such person, firm or corporation, however, shall be subject to the imposition under the provisions of this article of more than one tax by any such city on gross income or gross receipts for the privilege of doing business.