Why does John use a double “Verily, verily,” (John 1:53, et al.) when the other Gospel writers use only one?

"Verily I say unto you" is common in Matthew, Mark, and Luke, but John consistently uses the double "Verily, verily (Gk.amen, amen). (3:3, 3:5, 3:11, 5:19, 5:24, 25, 6:26, 32, 47, 53, 8:34, 51, 58, 10:1, 7, 12:24, 13:16, et ...