Oliver Cowdery initially wrote “as a great prophet and man of God” in the printer’s manuscript. Somewhat later, he supralinearly inserted a repeated indefinite article a before “man of God”. The ink flow is slightly heavier and definitely broader, so there was some change in the quill prior to this correction. The correction was probably the result of proofing 𝓟 against 𝓞, no longer extant here. Since the initial reading is fully acceptable, Oliver’s only motivation for inserting the a in 𝓟 would have been to make sure that 𝓟 read like 𝓞.
There are two other examples involving the indefinite article a where the noun prophet is conjoined with another noun, and in both cases the a is repeated, just like here in Helaman 11:18:
The original (and current) text of the Book of Mormon has numerous examples of the repeated a in conjuncts of noun phrases; for a nearby example where the a was lost, see under Helaman 1:5 (although in that case the initial a, not the repeated a, was lost). For a complete list of the repeated a in the Book of Mormon text, see under conjunctive repetition in volume 3. For each instance, we follow the earliest textual sources in determining whether the indefinite article should be repeated or not.
Summary: Accept Oliver Cowdery’s insertion of the repeated a in Helaman 11:18 (“as a great prophet and a man of God”); the accidental loss of such repetition has occurred fairly often in the history of the text.