Repetitions have diverse uses in Scripture.
In prayer they argue affection.
In prophecy they note celerity and certainty.
In threatenings they note unavoidableness and suddenness
In precepts they note a necessity of performing them.
In truths, like that before us, they serve to show the necessity of believing and knowing them.
J. C. Ryle, Expository Thoughts on the Gospel of John, Vol. 1 (pg. 7).