The repeated subject pronoun he was omitted in the 1840 edition; the shortened reading continued in the RLDS textual tradition until the 1908 RLDS edition restored the he to the RLDS text, probably by reference to the printer’s manuscript.
The elimination of the subject pronoun he in the 1840 edition could be either a typo or an instance of intentional editing by Joseph Smith. The subject he could have been consciously deleted in order to avoid the awkward conjoining of two instances of he did so close to each other (“he did deliver them and he did shew forth his mighty power”). Yet elsewhere, we have quite a few examples of he did in close proximity, as in the following cases with three instances of he did:
The second example here shows that the repeated he can be accidentally omitted, at least initially (in this case by Oliver Cowdery as he copied from 𝓞 into 𝓟).
On the other hand, there are cases in the text where an initial he did is followed by and did rather than and he did:
And we can also get a mixture of the two possibilities:
These examples show that in each case we should follow the earliest textual sources in determining whether the subject pronoun he is repeated or not.
Summary: Retain the repeated subject pronoun he in Mosiah 23:24 (“he did deliver them and he did shew forth his mighty power”).