Here the 1830 typesetter omitted the word once. It was restored to the text by reference to the manuscripts, 𝓟 for the 1908 RLDS edition and probably both 𝓞 and 𝓟 for the 1981 LDS edition. In this passage the word once seems to mean ‘formerly’ or ‘in the past’. The text is not emphasizing a specific moment or instance of rejection (where once would mean ‘one time only’). Note, for instance, the similar language in the King James Bible in reference to Paul’s conversion: “he which persecuted us in times past now preacheth the faith which once he destroyed” (Galatians 1:23). There are similar uses in the Book of Mormon text of once with the meaning ‘formerly’, as in the following sampling:
The unexpectedness of once in Alma 54:8 may have led the 1830 typesetter to omit the word, although probably unintentionally.
Summary: Maintain the original occurrence of once in Alma 54:8; the word here has the meaning ‘formerly’ rather than the more etymological meaning ‘one time only’.