The appositive use of “a mountain” is not an error in the original manuscript. Oliver Cowdery omitted “a mountain” when he copied the text from 𝓞 into 𝓟. Perhaps his eye skipped from the first to the second mountain as he copied. Another possibility is that he thought the appositive usage was either unnecessary or a scribal error.
There are numerous other uses of appositives in the Book of Mormon text, such as the following example where the appositive repeats “the land”:
Note here that the appositive “the land” could have been deleted without causing problems in interpreting the sentence. The appositive usage in the original manuscript for 1 Nephi 11:1 is probably not a case of dittography since there is no exact repetition (“an exceeding high mountain” is sufficiently different from “a mountain”). Thus the earliest reading with the appositive usage should be restored.
Summary: Restore the appositive “a mountain” in 1 Nephi 11:1 since this usage doesn’t appear to be a dittography; this kind of appositive is supported by other examples in the text.