Q. What if he (one being paid for reciting kadish – see prior question 802) has to say kadish for his own parent?

A. Chelkas Ya’akov (3: 157) rules that one is allowed to accept the kadish recitation for someone else at the same time he is already saying for his parent. He recommends to use part of the money received to purchase additional candles for the benefit of the soul of his departed father. However, he advises not to continue reciting into the twelfth month after the demise of his parent, as that would be insulting to the memory of his progenitor. (See Kol Bo on Avelus p.376, Mourning in Halacha p.363).
Horav Shlomo Miller’s Shlit”a opinion is that the obligation of a son is to recite at least one kadish per tefilah. If he has complied with that, any additional available kadesishim could be said for other deceased.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a