Definition of a Brahmana!!!

[Chandran, Nanda (NBC)]

I came across this yesterday :

King Janaka, an acknowledged jnani, in a dispute between the Brahmanas
and the Kshtriyas, invited a Brahmana to compete against him in open
debate (I find this itself strange, since a jnani is supposed to be
devoid of an Ego!). But the sage Uddalaka cautioned the other Brahmanas
thus : "If I enter into a debate with him (Janaka) and he defeats me,
then they will say that a Kshtriya defeated a Brahmana. But if I defeat
him, what shall I say, "I, a Brahmana, defeated a Kshtriya?". There's no
point in such a debate.".

Rather contradicts the point that one who attains Brahman is a Brahmana!

Can somebody give some information as to where this text occurs?

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