Apparently the editors for the 1920 LDS edition did not like the conjunction but reversing a previous but, so they changed the second but here in Alma 42:30 to and (the change is marked in the 1920 committee copy). It is possible that the second but was originally an and that was accidentally replaced by but when Joseph Smith dictated the text to Oliver Cowdery (both but’s are extant in 𝓞). There is evidence that Oliver Cowdery sometimes mixed up but and and in 𝓞 and in 𝓟; see the list of examples under 1 Nephi 8:20 (in the discussion regarding straight versus strait ).
Yet the original text has other examples where a but-initial clause follows a previous but; in the following examples, the second but is basically equivalent to ‘and’ and could, in fact, be emended to and without changing the meaning:
In these examples, the second but extends the scope of negation. For a similar example where but extends the scope of negation, see under Alma 43:20.
Since the Book of Mormon text allows a repeated but to have the meaning ‘and’, the critical text will restore the repeated but here in Alma 42:30 (“but let it bring you down to the dust in humility”).
Summary: Restore the original conjunctive repetition of but in Alma 42:30; occasionally the original text has instances where the repeated but means simply ‘and’ (that is, without reversing the previous but).