There has been a strong tendency in the history of the text to accidentally omit a repeated indefi- nite article when conjoining adjectives. As noted under 1 Nephi 11:35, many conjoined adjectives in the Book of Mormon text have a repeated indefinite article. Here in 1 Nephi 12:18, the original text read “a great and a terrible gulf”. The 1858 Wright edition accidentally dropped the repeated a, as did the 1906 LDS large-print edition. (The 1874 and 1892 RLDS editions followed the 1858 Wright text.) We shall see many examples of this tendency to lose the repeated a in the history of the text, mainly because speakers of modern English do not expect such repetition. For a list of examples, see conjunctive repetition in volume 3.
The original text, however, is not particularly systematic about whether the a should be repeated or not, even for specific adjectives. For instance, when great and terrible are combined, the a is not repeated except in 1 Nephi 12:18:
In each case, we follow the earliest textual sources, which means that in 1 Nephi 12:18, the a is repeated.
Summary: Maintain the repeated a in 1 Nephi 12:18 since this is the reading in 𝓞; such repetition occurs fairly often in the text.