Here the 1920 LDS edition accidentally, it would appear, replaced the plural abominations with the singular abomination. This change was not marked in the committee copy for that edition (a copy of the 1911 LDS Chicago edition), which implies that this 1920 change was unintentional.
Elsewhere in the text, we get considerable variety in the number for the conjoined nouns iniquity and abomination:
iniquity and abomination:
iniquity and abominations:
iniquities and abominations:
This variety argues that in each case we should follow the earliest extant reading. Thus the critical text will restore the plural abominations in Alma 13:17.
Summary: Restore in Alma 13:17 the original reading “iniquity and abominations”, which is supported by usage in two other places in the text.