Here the 1902 LDS missionary edition accidentally omitted the repeated indefinite article a. As discussed under Alma 9:5, the repeated determiner is expected in expressions referring to “hard-hearted and stiff-necked people”. For a more general discussion, see under conjunctive repetition in volume 3.
Summary: Maintain the repeated a in Alma 20:30: “into the hands of a more hardened and a more stiff-necked people”.