The 1852 LDS edition accidentally omitted the repeated indefinite article before mighty man here in Omni 1:28. The resulting reading has continued in the LDS text since then. The Book of Mormon text has numerous examples of the repeated indefinite article with conjoined adjectives, such as “a great and a terrible gulf ” in 1 Nephi 12:18 (see the discussion under that passage). For a general discussion, see conjunctive repetition in volume 3.
Such repetition of the indefinite article a is not required, but it is a prominent characteristic of the Book of Mormon text. Elsewhere, there are two similar instances of “strong and mighty”, one of which repeats the a:
In each case, we follow the earliest textual sources in determining whether the indefinite article is repeated.
Summary: Restore the repeated indefinite article a in Omni 1:28 (“a strong and a mighty man”).