In close succession, we get five occurrences of the phrase “saith the Lamb”. We get the longer “saith the Lamb of God” both before and after this passage (see 1 Nephi 13:33, 34 and 1 Nephi 14:3, 7). Joseph Smith, in his editing for the 1837 edition, removed one of the five occurrences of “saith the Lamb” (the second one listed above), perhaps because he felt that “saith the Lamb” was too frequent in this passage. Nonetheless, he kept the third and the fourth occurrences, even though both of these are even closer to each other than the first and second occurrences, so perhaps there was some other motivation for his deletion of the second occurrence. The critical text will, of course, restore the second occurrence.
Summary: Keep all five occurrences of “saith the Lamb” in 1 Nephi 13:34–36; this successive use of the same shortened version of “saith the Lamb of God” is clearly intended.