Initially in the printer’s manuscript, Oliver Cowdery seems to have written a dittography, “nay nay”, which he corrected by deleting the first nay. The correction appears to be based on the reading in 𝓞, which is no longer extant for this passage. (It is also possible that Oliver simply rewrote the word nay because the first one involved a scribal slip: his first nay looks like it was initially written as ray.)
Elsewhere the Book of Mormon text repeats nay only once, and that occurs in a biblical quotation from the King James Bible:
In fact, this is also the only passage in the entire Book of Mormon text where yea is repeated. Thus the introduction of “nay nay” in 2 Nephi 26:26 most probably involves a scribal error that Oliver Cowdery immediately corrected.
Summary: Accept in 2 Nephi 26:26 the corrected reading in 𝓟 that has only a single nay (“behold I say unto you nay”).