The 1907 LDS vest-pocket edition inserted the infinitival to before the verb exhort. The 1911 LDS edition also added this extra to, perhaps independently. Subsequent LDS editions have followed this usage since it seems so appropriate given the otherwise systematic use of the infinitival to throughout the passage (“to preach ... or to pray or to supplicate or to sing”).
The repeated to is possible but not necessary, as seen in the following pair of nearly identical examples involving whether; the first has the repeated to, the second does not:
These two examples from Alma involve a choice between two options, good and evil. But in Moroni 6:9, there are five options, each of which has the repeated to except for exhort. One could argue, however, that the to does not occur with exhort because it essentially means the same as the preceding preach. But this would not explain why the to is then repeated for the conjunctive pair “to pray or to supplicate”, since both pray and supplicate are semantically close.
When we look at the history of the text, we find there are seven cases in the printed editions where a repeated infinitival to has been lost:
Oliver Cowdery also tended to omit the repeated to. Here are three cases in 𝓟 where he initially omitted the repeated to but corrected his error; in each case, 𝓞 is extant and has the repeated to:
Despite this strong tendency to omit the repeated to, there is one firm case in the manuscripts where Oliver Cowdery persisted in adding an intrusive to:
In this case, Oliver started to write the repeated to in 𝓞, but then he immediately erased it and overwrote the abraded to with the beginning of the following word, put. Yet when he came to copying this infinitival clause into 𝓟, he reinserted the repeated to, this time without correcting it, thus writing in 𝓟 “to keep them or to put them to death”. This example shows that Oliver Cowdery could insert a repeated to as well as delete it.
Ultimately, the safest solution here in Moroni 6:9 is to accept the reading of the earliest textual sources—namely, without any repeated to (thus “whether to preach or exhort”). The critical text will therefore remove the repeated to since it is lacking in the earliest extant source, the printer’s manuscript.
Summary: Restore in Moroni 6:9 the earliest reading, which lacks the repeated to in “whether to preach or exhort”.