It depends on the context but both have their uses and are both commonly used. "He keeps on _repeating [it] again and again_." This could be in reference to someone constantly repeating a behaviour or habit, generally in reference to something annoying or constant. Whereas, "We keep on _go[ing] over it again [and again]_." would be good to use in regards to having to constantly tell someone about something they should do or change. Both sound natural to native speakers of English. Good job and I hope this helps!

I would say, "repeat (something) over and over" and "go over (something) again and again"