Section 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.