It is possible that the earliest text for this passage contains a dittography. We do not have the original manuscript here, but the identical repetition of the text (compare the second if- clause with the third one) suggests that a whole line might have been accidentally repeated by Oliver Cowdery when he copied the text from 𝓞 into 𝓟. The length of the repeated portion (“if it so be that they harden not their hearts against the Lamb”) agrees with the length of the line that scribe 2, the scribe here in 𝓞, typically wrote down in the original manuscript (about 60 characters, including spaces), so Oliver’s eye might have skipped up one line, which would have led to a dittography of a line’s length.
Joseph Smith, in his editing for the 1837 edition, did not apparently like the repetition, so he decided to conjoin the second if- clause with the first one (“if the Gentiles shall hearken … ”) by deleting the initial words of the second if-clause (“if it so be that they”) and by adding the conjunction and. He then deleted the third if- clause, but he wanted to retain the phrase “of God” that was at the end of the third if- clause, so he supralinearly inserted God at the end of the nowtruncated second if- clause. The 1837 edition correctly interpreted Joseph’s intentions here and set “the Lamb of God” rather than “the Lamb God”.
For the 1849 LDS edition, Orson Pratt apparently thought the truncated second if-clause was too abrupt, so he inserted “if they”after the and. But this addition to the text was removed in the 1920 LDS edition, undoubtedly by reference to an earlier edition (such as the 1837 or 1840 edition). The only substantive issue here is whether Oliver Cowdery created a long dittography. Yet such a dittography would be uncharacteristic of the scribal dittographies found in the two manuscripts: these dittographies are at most only a few words long. (A few whole-line dittographies are found in the printed editions but not in the manuscripts.) In addition, the original use of the conjunction and before the third if- clause suggests that the repetition is intended rather than accidental.
Obviously, the earliest text does work. In fact, there is one example of an if-clause occurring at the end of a sentence which is then followed by another sentence that begins with the semantic equivalence of restating the same condition as the previous if-clause. In this particular example, the second if-clause uses the adverbial so to show the repetition of the condition rather than explicitly repeating the words:
Ultimately, there is no real evidence besides the repetition itself that the repeated if-clause in 1 Nephi 14:1–2 is due to scribal error. Both clauses were apparently in the original text, even though the result involves redundancy or wordiness.
Summary: Restore the earliest text in 1 Nephi 14:1–2, where a following if-clause repeats the preceding if-clause nearly word for word; there is nothing inherently wrong with this repetition in the earliest text.