Here in the printer’s manuscript, Oliver Cowdery initially wrote “among the Lamanites or the Nephites”; then he supralinearly inserted the repeated among (with no change in the level of ink flow). His correction undoubtedly reflects the reading of the original manuscript, especially since in English we expect the preposition to not be repeated. His error was probably prompted by the lack of repetition in the parallel sentence in the preceding verse:
The repeated among is characteristic of the Book of Mormon text and occurs with other instances of conjuncts conjoined by or:
But there are other cases without the repeated among:
(Note that all five of these other cases involve the corrective or, not the regular disjunctive or used twice in Helaman 6:7–8.)
So either reading, with or without the among, is theoretically possible. In each case, we follow the earliest reading; thus the repeated among will be maintained in Helaman 6:8. (We can also find evidence for the repeated among when the conjunction is and rather than or; see under Alma 51:7 for three examples.)
Summary: Accept Oliver Cowdery’s inserted among in Helaman 6:8, the corrected reading in 𝓟, as the reading of the original text (and presumably the reading in 𝓞).