Here Oliver Cowdery inserted the repeated preposition by in the printer’s manuscript. The level of ink flow is unchanged for the supralinear by, so the correction appears to be virtually immediate. The corrected reading in 𝓟 was most likely the reading in 𝓞, no longer extant here.
In this case, the expression does seem more natural when the by is repeated after the negative conjunction nor. In any event, such parallelism is characteristic of the Book of Mormon text, although it is not required. See the discussion under Alma 2:38 and, more generally, under conjunctive repetition in volume 3.
Summary: Accept in Helaman 6:22 Oliver Cowdery’s inserted by in 𝓟 (“he should not be injured by his brother nor by those who did belong to his band”); repetition of the preposition in conjunctive structures is common in the Book of Mormon text.