Jesus uses the expression "verily, verily" (usually followed by "I say unto you") 25 times in the New Testament, all in the Gospel of John. One such example:
Verily, verily, I say unto you, He that receiveth whomsoever I send receiveth me; and he that receiveth me receiveth him that sent me. (John 13:20)None of the Synoptic Gospels ever quote Him thus, and nowhere else in the Bible is this expression used by Him or anyone else.
"Verily, verily" also appears 25 times in the Book of Mormon. Here, too, the resurrected Lord--during his ministry among the Nephites in 3 Nephi--is the sole user of this expression (with one exception, Alma 48:17). Eight of these instances are found, oddly, in the Savior's revisitation of the Sermon on the Mount (chapters 12-14 of 3 Nephi). Remember, the Biblical account of the Sermon is not contained in the Gospel of John, but Matthew, where "verily, verily" is nowhere, nowhere to be found. One comparison to illustrate:
Verily, verily, I say unto you, I give unto you to be the light of this people. A city that is set on a hill cannot be hid. (3 Nephi 12:14)
Ye are the light of the world. A city that is set on an hill cannot be hid. (Matthew 5:14)Isn't it interesting that, when plagiarizing the Sermon on the Mount from Matthew, Joseph would throw in a bit of language exclusive to another Gospel? Sarcasm aside, I wonder if this isn't the only example of the "Fifth Gospel" amalgamating some of the other Four.
Side note: "Verily, verily" also appears in the Doctrine and Covenants, and more often than in the Bible and Book of Mormon combined. Again, only the Lord, with Joseph as mouthpiece, uses this expression (or at least I think so; I'm too lazy to go through 50+ verses to verify that).
UPDATE (12/17/12): Krister Stendhal, the rare non-Mormon biblical scholar to find value in textual analysis of the Book of Mormon, noticed this, too. He cited this conflation as but one example of 3 Nephi portraying a Jesus more consistent with the one found in the Gospel of John ("the revealed revealer who points to himself and to faith and obedience to him as the message") than the Jesus of the synoptic Gospels (a "teacher of righteousness") . The effect of this insertion of "Johannine" language in particular, he says, is not to be understated:
"...let me point to [a] feature that must strike us all. It is one of style. I refer to the abundance of the introductory words “verily” and “verily, verily,” the Greek and Hebrew “amen.”...By this stylistic device the teaching of Jesus is actually changed from moral and religious teaching into proclamation and explicit revelation of divine truth. The whole speech has thereby changed its character."The subtle inclusion of this phrase, then, is reflective of one of the Book of Mormon's key purposes: "For behold, this [the Book of Mormon] is written for the intent that ye may believe that [the Bible] (Mormon 7:9);" the Book of Mormon affirms the Sermon to be not merely wisdom from a great moral teacher, but divinely revealed truth from a loving God.
 Krister Stendahl, “The Sermon on the Mount and Third Nephi,” in Reflections on Mormonism: Judaeo-Christian Parallels, ed. Truman G. Madsen (Provo, UT: Religious Studies Center, Brigham Young University, 1978), 139–54. Online here. Citied in Givens, Terryl L. (2002-03-14). By the Hand of Mormon: The American Scripture that Launched a New World Religion (p. 138). Oxford University Press.