Here in the printer’s manuscript, as Oliver Cowdery copied from the original manuscript (no longer extant for this part of the text), his eye skipped from the word astonishment back to the similar astonished, with the result that initially in 𝓟 he repeated after astonishment the earlier that-clause, “that we fell to the earth”. Oliver immediately caught his error, crossed out the dittography, and wrote the correct text by inserting “behold they cast us into” above the crossedout text and finishing up by writing inline the last word in the sentence, prison. The immediacy of the correction shows that the corrected reading in 𝓟 was undoubtedly the reading in 𝓞. Moreover, the dittography does not make any sense.
Summary: Maintain in Helaman 9:14 the corrected reading in 𝓟, where the verse ends with the main clause, “behold they cast us into prison”, rather than with the nonsensical initial repetition in 𝓟 of the subordinate that-clause (“that we fell to the earth”).