Adi Shankara says
"By the words 'One only' is meant that there was nothing else coming under the catergory of Its product. By the words 'without a second' this is meant: As in the case of pot, etc. some other efficient causes like potters and others, who are different from earth etc. into pot etc. are seen, similarily (here) also there arises the possibility of having some other second thing which is different from Existence, and yet is a cause associated with Existence. This is being denied by the phase, 'without a second (advitiyam)'. So, 'without a second' means that It (Existence) has …