Here in the first printing of the 1852 LDS edition, there were two typos that were removed in the second printing, probably by reference to the 1840 edition. First, a repeated on was inserted before his breastplate, prompted undoubtedly by the on before the immediately preceding his headplate. Second, the plural shields was changed to shield, either because of the preceding singular headplate and breastplate or, more likely, because the 1852 typesetter expected Moroni to have but one shield.
Elsewhere in the Book of Mormon text, we have only plural uses of shields, and the reference is always to groups of warriors and not a single individual as here in Alma 46:13. Nonetheless, when we look at these other occurrences, we discover that a warrior, in full armor, would have had various shields protecting at least his head, chest, and arms:
There were breastplates, armshields, bucklers, and headplates. In Alma 43:19, headplates are virtually defined as “shields to defend their heads”. In Helaman 1:14, we have a generic reference to “all manner of shields of every kind”. It appears that there were different kinds of shields that an individual warrior could put on; thus the use of shields in Alma 46:13 is perfectly correct.
Summary: Maintain in Alma 46:13 the plural use of shields in reference to Moroni’s armor since shields can refer to armshields, headplates, and “all manner of shields of every kind”; the preposition on is not repeated in the conjunctive prepositional phrase, “on his headplate and his breastplate and his shields”.