The repetition of and he did in the middle of this passage appears to be an accidental dittography that entered the text when Oliver Cowdery copied this passage from 𝓞 into 𝓟. The current text is excessively redundant, as we can clearly see when we supply the ellipsis:
Without the extra “and he did”, we get a more parallelistic statement that avoids any excessive redundancy:
Of course, the resulting parallelism is quite repetitious, as pointed out by David Calabro (personal communication). Even so, the first line refers to what Moronihah commanded Lehi to do, and the second line confirms that Lehi did precisely that. We get a similar kind of repetition, although not as parallelistic, later in the text when Gidgiddoni commanded his armies and they followed through:
Here in Helaman 1, the original manuscript is not extant for the first part of verse 29, but spacing between extant fragments indicates that most likely Oliver simply wrote “& thus he did head them” in 𝓞; the transcript for this part of 𝓞 in volume 1 of the critical text reads accordingly:
If the extra “& he did” was in 𝓞, Oliver must have supralinearly inserted it, as follows:
Elsewhere in the text there are 33 occurrences of “and thus did < do something >”, as in the following sampling:
However, there is one clear example in the original text of “and thus did” where the main verb is ellipted:
Providing the ellipted text, we get “and thus they did march forth into the city and take possession of it”, which causes no excessive redundancy. (For another possible example of “and thus they did”, see the discussion under Alma 24:18.)
Although there is theoretically nothing wrong with “and thus he did” in Helaman 1:29, the redundancy is excessive. In addition, spacing considerations in 𝓞 argue against the earliest extant reading (in 𝓟), which is also the reading in all the printed editions. Here in Helaman 1:29, the critical text will accept the emended reading without the dittography. As explained under Mosiah 10:5, there are several cases of dittography that appear to have entered the text during its early transmission. Each one involves an unacceptable sort of redundancy.
Summary: Emend Helaman 1:29 by removing the apparent dittography (“and he did”), thus giving “and thus he did head them”.