In this adjectival conjunctive phrase, the repeated a was lost in the 1840 edition as well as in the 1852 LDS edition. The 1908 RLDS edition restored the original repeated a, most probably by reference to 𝓟. The LDS text has continued with the secondary reading that omits the repeated a.
In general, the indefinite article is repeated for the conjoined adjective in “a large and (a) ”, although interestingly the clear tendency in the history of the text has been to omit that repeated article at least once during the textual history for every example of this construction that originally had the repeated a. Besides the example in Mosiah 27:7, we have these three other examples:
On the other hand, when large is conjoined with spacious, the a is never repeated in the original text:
In fact, whenever any adjective is conjoined with spacious, no determiner (such as a, the, or many) is ever repeated. For a list of these other examples involving spacious, see under 1 Nephi 11:35. For a general discussion, see under conjunctive repetition in volume 3.
Summary: In accord with the reading of the earliest textual sources, restore the repeated a in Mosiah 27:7: “and they became a large and a wealthy people”.