Here the printer’s manuscript does not repeat the again, but the 1830 edition does. The use of the repeated again does sound strange, and this could explain why Oliver Cowdery might have dropped it when copying the text into 𝓟. But it is also possible that the 1830 typesetter added the again simply because it occurred in the previous clause (“and the robbers did not come again to battle”).
There are a few other times in the Book of Mormon text where again occurs in a negative clause referring to battle:
There is also considerable evidence that Oliver Cowdery occasionally added or deleted again when he copied the text from 𝓞 into 𝓟. For instance, in 1 Nephi 11:30–36 Oliver added again to the text three times, probably because he did not trust what the unidentified scribe 3 of 𝓞 had written while taking down Joseph Smith’s dictation (see the discussion under 1 Nephi 11:30 for all three of those cases). But more significant are five cases where Oliver, in his copying, initially omitted an again, then somewhat later corrected the text by supralinearly inserting the again:
The case in Mosiah 10:21 is particularly relevant here because it has a repeated again (just like here in 3 Nephi 4:15); in other words, Oliver sometimes eliminated repeated again’s. On the other hand, the 1830 typesetter never once added or deleted an again. Thus the evidence argues that in 3 Nephi 4:15 𝓞 probably had the repeated again and that Oliver accidentally omitted it when he copied from 𝓞 into 𝓟.
Summary: Maintain in 3 Nephi 4:15 the repeated again despite its strangeness; Oliver Cowdery was prone to omit again in his copywork, although he normally caught this error.