The Book of Mormon includes a Hebraism known as the emphatic pronoun. This repetition of the person pronoun is a deliberate emphasis which can be seen as redundant in English.
Donald W. Parry explains:
For purposes of emphasis, biblical Hebrew sometimes repeats the personal pronoun. This usage, termed the “emphatic pronoun,” occurs when the pronoun is the subject, as in Genesis 6:17, where the Lord states, “Behold, I, even I, do bring a flood of waters upon the earth” (emphasis added); or when the pronoun is the object, as in Genesis 27:38, where Esau implores his father to “bless me, even me also, O my father” (emphasis added). Some translators do not translate the emphatic pronoun, perhaps considering it unnatural or simply redundant in English.
The Book of Mormon also has examples of the emphatic pronoun. King Benjamin, speaking to a Nephite multitude, says, “And I, even I, whom ye call your king, am no better than ye yourselves are” (Mosiah 2:26; see v. 4)– Donald W. Parry, “Hebraisms and Other Ancient Peculiarities in the Book of Mormon,” in Echoes and Evidences of the Book of Mormon
Another example is in Mosiah 10:10
And it came to pass that we did go up to battle against the Lamanites; and I, even I, in my old age, did go up to battle against the Lamanites. And it came to pass that we did go up in the strength of the Lord to battle.
How did Joseph Smith know about the emphatic pronoun if most people probably do not notice it in the Bible? How would Joseph remember to include it while dictating the Book of Mormon?