Verse 18 is oddly parallel to verse 17, and appears to be deliberately so. It appears to require a second “Moses” and therefore a second “spokesman.” Grammatically, one would hope for a direct object in the sentence “I will raise up unto the fruit of they loins...” The missing object must be “another” in order to make the parallelism of the phrases work.
In any case, this has been the traditional interpretation of verse 18, with the “spokesman” of the second, parallel, Moses being identified as Sydney Rigdon. “Many members of the Church believe that Sidney Rigdon is the ”spokesman“ referred to in this prophecy. It appears that Joseph Smith is the prophet mentioned in this verse, and the Lord promised Sidney Ridgon that he should ”be a spokesman to my servant Joseph.“ (D&C 100:9)” (Ludlow, Daniel H. A Companion to your Study of the Book of Mormon. Desert Book 1976, p. 129).