Here the 1840 edition deleted the repeated their in the phrase “their pride and their nobility”. Yet as we have seen many times, the Book of Mormon text prefers the repeated determiner in conjoined constructions. Undoubtedly this change in the 1840 edition is not due to Joseph Smith’s editing for that edition since the same phrase in the next verse retains the repeated their: “and they did pull down their pride and their nobility” (Alma 51:18). The repeated their in Alma 51:17 was restored to the RLDS text in 1908. For further examples of the loss of the repeated their, see under conjunctive repetition in volume 3.
Summary: Maintain in Alma 51:17 the repeated their in “their pride and their nobility”; the 1840 omission of the their is most probably a typo rather than the result of editing on the part of Joseph Smith.