The tendency in English is to not repeat identical items in conjuncts. Here the original text repeats the preposition unto (“unto Laman and unto Lemuel”). Oliver Cowdery, in copying from 𝓞 to𝓟, accidentally dropped the repeated preposition, but then he caught his error and supralinearly inserted the second unto. The text has numerous examples of the repeated preposition, plus considerable evidence of the difficulties in transmitting such repetition.
Prepositional repetition is characteristic of the biblical Hebrew and Greek and is therefore found throughout the King James Bible, a literal translation in this respect:
Such multiple repetition of unto is characteristic of the Book of Mormon:
Summary: Maintain the repetition of prepositions wherever supported by the earliest textual sources; such repetition is a characteristic of the biblical style but not of standard English.