Isaiah 8:19 (King James Bible) and unto wizards that peep and that mutter
The Book of Mormon omits the repeated relative pronoun that in 2 Nephi 18:19; the resulting reading is more natural in English. It is difficult to determine whether the original text itself omitted the repeated that since it is quite possible that the that was accidentally deleted during the early transmission of the text. Either alternative seems plausible.
There is another Isaiah passage where a repeated relative pronoun that was not omitted. In this instance, the Book of Mormon quote is partially paraphrastic:
When compared with 2 Nephi 18:19, the conjoined relative clauses in 2 Nephi 27:3 are longer (“that fight against Zion and that distress her”). Even so, the Book of Mormon text in 2 Nephi 27:3 would more easily read without the repeated that: “that fight against Zion and distress her”.
Ultimately, since the Book of Mormon reading is perfectly fine, the most reasonable solution is to accept the reading of the earliest textual source, “that peep and mutter”.
Summary: Accept in 2 Nephi 18:19 the reading in 𝓟 (“that peep and mutter”), which flows more easily than the King James expression (“that peep and that mutter”); nonetheless, it is possible that the omission of the repeated that was accidental.