स यथा तत्र नादाह्येतैतदात्म्यमिदं सर्वं तत्सत्यं स आत्मा तत्त्वमसि श्वेतकेतो इति तद्धास्य विजज्ञाविति विजज्ञाविति ॥ ६.१६.३ ॥
In plain words
And as in that case he is not burnt, so this whole world has that as its self. That is the real. That is the Self. That thou art, Śvetaketu." And he understood this from him — he understood it.
The commentators
Chāndogyopaniṣadbhāṣyam· bhāṣya
प्रतिबिम्बरूपेण । स आत्मानं कार्यकरणेभ्यः प्रविभक्तं सद्रूपं सर्वात्मानं प्राक् पितुः श्रवणात् न विजज्ञौ । अथेदानीं पित्रा प्रतिबोधितः तत्त्वमसि इति दृष्टान्तैर्हेतुभिश्च तत् पितु- रस्य ह किलोक्तं सदेवाहमस्मीति विजज्ञौ विज्ञातवान् । द्विर्वचनमध्यायपरिसमाप्त्यर्थम् ॥
In plain words
As a mere reflection. Before hearing from his father, he did not recognize the Self as distinct from the body and senses, existing as the absolute reality and the self of all. But now, awakened by his father through the teaching 'Thou art That,' along with examples and logic, he has realized the truth spoken by his father: 'I am Being itself.' He has truly known. The doubling of words marks the end of the chapter.
Close to the Sanskrit
As a reflection. He did not know the Self—distinct from body and instruments, of the nature of being, the all-self—before hearing from the father. Now, awakened by the father through 'Thou art That' and through examples and reasons, he indeed realized that he is existence itself, as spoken by this father; he understood. The repetition is for the purpose of concluding the chapter.