Here the 1852 LDS edition accidentally omitted the repeated indefinite article a before fallen. For two other instances where the text has accidentally lost the repeated a in the phrase “a lost and a fallen people”, see the discussion under Alma 9:30, 32. Also see the general discussion under conjunctive repetition in volume 3 .
Summary: Restore in Alma 12:22 the repeated a in “a lost and a fallen people”.