According to Wade Brown, on over 100 occasions Book of Mormon prophets expressed their beliefs by using such phrases as "I know." But Alma put special emphasis on his expression by stating "I do know." Alma used the word combination "I do know" a total of eight times in four entirely separate sections (Alma 5:46; 7:8; 34:8; 36:3, 26, 30; 40:9).
No one else ever used it--except--his missionary junior companion Amulek. After working with Alma and hearing him say it many times, Amulek repeated the phrase "I do know" once himself. [C. Wade Brown, The First Page of the Golden Plates, p. 29]
Note* It is interesting that King Lamoni, in his search for the truth, uses the phrase "I do not know" (Alma 18:25, 29) in addressing Ammon. It is also intriguing that in Alma's condemnation to the anti-Christ Korihor he uses the phrase "ye do not know" twice (Alma 30:24, 26). In his final verbal assault before he was struck dumb, Korihor says the following to Alma: "I do not deny the existence of a God, but I do not believe that there is a God; and I say also, that ye do not know that there is a God; and except ye show me a sign, I will not believe. [Alan C. Miner, Personal Notes] [See the commentary on the covenant phrase "I know" in 1 Nephi 3:7; 5:2, 5:5; 2 Nephi 33:4; 3 Nephi 14:23]