Real Bible Translation

Themselves, yourselves, bear witness to myself that I spoke, "Myself is not myself,^70f the Anointed One, but that I am the one who has been sent away in front of that one.

LITV Translation:
You yourselves witness to me that I said, I am not the Christ, but that having been sent, I am going before that One.

ESV Translation:
You yourselves bear me witness, that I said, ‘I am not the Christ, but I have been sent before him.’

Footnotes

70f

Greek ὅτι εἶπον ἐγώ οὐκ εἰμὶ ἐγώ. that I spoke I am not I.

The most textual critical editions of the Greek NT based on the earliest manuscripts. Later copies of the NT perhaps considered that 'ego' was repeated accidentally and removed it. In the most recent modern critical editions the word "because/that" is placed in between brackets [ὅτι]. See both the NA28 and UBS5 edition on this verse. Thus, the reader is presented with two choices of readings:

"that I said that" or "that I said I am not I". Which makes less sense? "That I said that" is most certainly redundant and makes no sense as ὅτι is a demonstrative conjunction, and is not what is found in the earliest manuscript evidence.

Κατὰ Ἰωάννην 3:28 Greek NT: Westcott and Hort 1881
αὐτοὶ ὑμεῖς μοι μαρτυρεῖτε ὅτι οὐκ εἰμὶ ἐγώ ὁ Χριστός, ἀλλ' ὅτι Ἀπεσταλμένος ἐκεῖνος ἐκείνου ἐγὼ οὐκ εἰμὶ ἔμπροσθεν ἐκείνου.

Κατὰ Ἰωάννην 3:28 Greek NT: Westcott and Hort / [NA27 and UBS4 variants]
αὐτοὶ ὑμεῖς μοι μαρτυρεῖτε ὅτι οὐκ εἰμὶ ἐγώ / ὅτι οὐκ εἰμὶ ἐγώ ὁ Χριστός, ἀλλ' ὅτι Ἀπεσταλμένος ἐκεῖνος ἐκείνου ἐγὼ οὐκ εἰμὶ ἔμπροσθεν ἐκείνου.

The Westcott and Hort edition and subsequent critical editions (Nestle-Aland and United Bible Societies' editions) prioritized the examination of the earliest manuscripts for reconstructing the original text of the New Testament.

The reasoning behind this approach is that earlier manuscripts are generally considered to be closer in time to the original writings of the New Testament authors and are thus more likely to preserve the original wording accurately. Therefore, these critical editions typically give more weight to readings found in earlier manuscripts when determining the most likely original text.

What this tells us is that the "extra" word ἐγώ ego (I/Myself) had been removed in many editions:

Κατὰ Ἰωάννην 3:28 Greek NT: RP Byzantine Majority Text 2005
... ὅτι εἶπον, οὐκ εἰμὶ ἐγώ ...

Κατὰ Ἰωάννην 3:28 Greek NT: Greek Orthodox Church
... ὅτι εἶπον· οὐκ εἰμὶ ἐγώ ...

Κατὰ Ἰωάννην 3:28 Greek NT: Tischendorf 8th Edition
... ὅτι εἶπον· οὐκ εἰμὶ ἐγώ ...

Κατὰ Ἰωάννην 3:28 Greek NT: Scrivener's Textus Receptus 1894
... ὅτι εἶπον, οὐκ εἰμὶ ἐγώ ...

Κατὰ Ἰωάννην 3:28 Greek NT: Stephanus Textus Receptus 1550
... ὅτι εἶπον, οὐκ εἰμὶ ἐγώ ...