Revelation 13:10

The phrases are doubled.

εἴ τις εἰς αἰχμαλωσίαν, εἰς αἰχμαλωσίαν ὑπάγει

τις ἐν μαχαίρᾳ ἀποκτενεῖ, αὐτὸν ἐν μαχαίρᾳ ἀποκτανθῆναι

And that was the point. The doubling of texts throughout the Hebrew and also Greek, in this manner, is obviously unconventional language, and does not resemble how any one speaks or communicates (unless they stutter). But in a timeless two-sided realm of duplicate reflections, unconventional language has to be used. In these cases, scholars found it better to change or alter texts rather than leave them as is.

The best attested reading:

• "If anyone into captivity, into captivity he goes; if anyone to be killed by the sword, he must be killed by the sword."
• This reading aligns closely with passages from Jeremiah 15:2 and Jeremiah 43:11, which emphasize the inevitability of divine judgment and preordained fates.
• The verb is not explicitly stated in the Greek text for the first clause, leaving it implied and adding to the interpretative challenge.

Cf. Cambridge Bible for Schools and Colleges, Commentary on Revelation 13:10