Here the 1874 RLDS edition omitted the repeated their from the conjunctive phrase “their leader and their chief commander”. This shortened reading has continued in the RLDS text, despite the fact that the printer’s manuscript contains the repeated their. Elsewhere the repeated their does occur in similar conjunctive phrases but not necessarily:
The their is conjunctively repeated once here (“their chief captain or their chief leader”, but it is not repeated in “their chief captains and leaders” or in “their chief leader and commander”). For each instance of possible repetition, we will follow the reading of the earliest text. For other examples where a repeated their has been omitted, see the discussion under conjunctive repetition in volume 3.
Summary: Maintain the repeated their in Alma 47:19: “the Lamanites appointed Amalickiah to be their leader and their chief commander”.