Here the original text repeats the preposition in. 𝓞 is not extant, but there is room for the repeated in in the lacuna. In any event, 𝓟 has the repeated in. The 1920 LDS edition removed it because of its redundancy. Nonetheless, such usage can be found in several other places in the original Book of Mormon text as well as in the King James Bible. For those examples and discussion, see under Jacob 7:8. The critical text will restore the repeated in here in Alma 55:16.
Summary: Restore in Alma 55:16 the repeated in since this is the reading of the earliest textual sources (in this case, 𝓟 and all the early editions); there is support for such usage elsewhere in the original text.