As discussed under 1 Nephi 16:38, the original text has a number of examples of the repeated a in cases of conjoined nouns, such as sign and wonder here in 3 Nephi 2:1. In this instance, the 1888 LDS edition omitted the repeated a; but since that edition never served as a copytext, subsequent LDS editions have maintained the a. Similarly, the a is repeated in the phrase “a marvelous work and a wonder”, which occurs twice in the Book of Mormon (in 2 Nephi 25:17 and 2 Nephi 27:26, where the text follows the King James language in Isaiah 29:14). The critical text will retain the repeated a here in 3 Nephi 2:1. For a general discussion of this usage, see under conjunctive repetition in volume 3.
Summary: Maintain in 3 Nephi 2:1 the repeated a in the conjoined noun phrase “a sign or a wonder from heaven”.