The 1837 edition accidentally deleted again here in 3 Nephi 2:18, probably because the immediately following word against is so similar. There is nothing wrong with saying that the robbers “did come forth again” since verse 17 describes how the people of Nephi “did drive them back out of their lands into the mountains and into their secret places”. Elsewhere the original text has four instances of again against, so there is no reason for deleting the again here in 3 Nephi 2:18. The 1908 RLDS edition restored the again to the RLDS text (most likely by reference to 𝓟); the LDS text has maintained the 1837 reading. For examples of other cases where the word again was lost in the 1837 edition, see under Helaman 15:13.
Summary: Restore the again in 3 Nephi 2:18 (“they did come forth again against the people of Nephi”); the 1837 edition accidentally omitted the again because of the immediately following word, the nearly identical against.