The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
35. As the Self is within all, as in the case of the aggregate of the elements, (there is oneness of vidyâ).
The Vâgasaneyins record, in the questions asked by Ushasta and by Kahola, the same passage twice in succession, 'Tell me the Brahman which is present to intuition, not hidden; the Self who is within all' (Bri. Up. III, 4, 1; 5, 1).--The question here presents itself whether the two sections introduced by the questions constitute one vidyâ only or two separate vidyâs.
Two separate vidyâs, the pûrvapakshin maintains; owing to the force of repetition. For if the second passage added nothing to--or took nothing away from--the contents of the first, the repetition would be altogether meaningless. We therefore conclude that the repetition intimates the separateness of the two vidyâs, just as in the Pûrva Mîmâmsâ repetition shows two sacrificial actions to be separate.
To this we make the following reply. As both texts equally declare the Self to be within all, they must be taken as constituting one vidyâ only. In both passages question and answer equally refer to a Self which is within everything. For in one body there cannot be two Selfs, each of which is inside everything else. One Self indeed may without difficulty be within everything, but of a second one this could not be predicated, not any more than of the aggregate of the elements; i.e. the case of that second Self is analogous to that of the aggregate of the five elements, i.e. the body. In the body the element of water is indeed within the element of earth, and the element of fire within the element of water; but each of these elements is 'within all' in a relative sense only, not in the literal sense of the phrase.--Or else the 'like the aggregate of the elements (or beings)' of the Sûtra has to be taken as pointing to another scriptural passage, viz. Sve. Up. VI, 11, 'He is the one god, hidden in all beings, all-pervading, the Self within all beings.' As this mantra records that one Self lives within the aggregate of all beings,
the same holds good with regard to the two Brâhmana-passages. And the object of knowledge being one, the vidyâ also is one only.