The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
36. If it be said that otherwise the separation (of the statements) cannot be accounted for; we reply that it is (here) as in the case of other instructions.
We yet have to refute the remark made by the pûrvapakshin that, unless the separateness of the two vidyâs be admitted, the separation of the two statements cannot be accounted for. We do this by pointing to analogous cases. In the sixth prapâthaka of the upanishad of the Tândins the instruction conveyed in the words, 'That is the Self, thou art that, O Svetaketu,' is repeated nine times, and yet the one vidyâ is not thereby split into many. Similarly in our case.--But how do you know that the vidyâ remains one and the same in spite of the ninefold repetition?--Because, we reply, the introductory and concluding clauses show that all those passages have the same sense. For the repeated request on the part of Svetaketu, 'Please, Sir, inform me still more,' shows that one and the same matter is again and again proposed for further discussion, and further instruction regarding it is repeatedly given by means of new doubts being removed. Similarly, in the case under discussion, the sameness of form of the two introductory questions and the equality of the concluding clauses, 'Everything else is of evil,' show that both sections refer to one and the same matter.--Moreover, in the second question the text adds the word: 'just' (eva), 'Tell me just that Brahman,' &c., which shows that the second question refers to the same matter as the first one. That the matter of the two sections is really the same, we establish by pointing out that the former section declares the existence of the highest Self which is neither cause nor effect, while the latter qualifies it as that which transcends all the attributes of the Samsâra state, such as hunger, thirst, and so on.--The two sections, therefore, form one vidyâ only.