As already discussed under 1 Nephi 2:14, Joseph Smith sometimes removed the auxiliary do when it was immediately followed by the main verb do. Here is one more example. The critical text will restore all these cases of archaic English. For a complete list, see do auxiliary in volume 3. Such usage is found in the biblical style, which dates from Early Modern English.
Summary: Restore the use of did do here in 2 Nephi 5:18 (“I did do for them”), the reading of the original text.