When he copied this part of the text from 𝓞 into 𝓟, Oliver Cowdery initially wrote the conjoined construction involving wars and contentions without repeating the determiner their. Shortly thereafter, he supralinearly inserted the repeated their (without any change in the level of ink flow). Normally the text does not repeat determiners in conjuncts involving wars and contentions:
Notice, in particular, the example found in the Helaman preface; there the determiner is not repeated for the immediately conjoined nouns wars and contentions, but it does occur with the following conjunct, dissensions (“the wars and contentions and the dissensions”).
There is one example in addition to Jacob 3:13 for which the determiner is repeated, but in this example the conjuncts wars and contentions are not immediately conjoined:
Thus the evidence is not particularly strong for repeating the determiner in the immediately conjoined wars and contentions. The expected phraseology is without any repeated determiner, which may explain why Oliver Cowdery initially wrote “their wars and contentions” in Jacob 3:13. Ultimately, there would have been little motivation for Oliver to have inserted the repeated their in this passage except that it must have occurred in the original manuscript. Therefore, the critical text will maintain the repeated their here in Jacob 3:13.
Generally speaking, we allow the earliest textual sources to determine in each case whether a determiner should be repeated for a given conjunct. For an example of a conjunctive structure where the determiner is usually not repeated, see “great and terrible” (under 1 Nephi 12:18). For the opposite possibility, where the determiner is usually repeated, see “statutes and judgments” (under 1 Nephi 17:22). For a general discussion of the repetition of determiners, see conjunctive repetition in volume 3.
Summary: In accord with the corrected reading in 𝓟, maintain in Jacob 3:13 the repeated their in “their wars and their contentions”, despite its unusualness.