I remeber when A. J. Burnett threw his no-hitter. These aren't exact numbers, but of the about 115 pitches he threw, about 105 were fastballs.
We had a little lefty here in St. Louis named Bud Smith who I believe threw a no-hitter also throwing only fastballs and change-ups.
The point is to locate the fastball and change speeds. Sometimes, if you locate the fastball with good enough velocity, you don't have to change speeds as much. In Burnett's case, it was grip and rip. I think he had about 7 or 8 walks in the game. His velocity, mid-90's made up for a lack of location. Smith was a constant change of speeds where he would throw the change two or three times in a row to keep guys off his fastball.
I, for the majority of pitchers, set a goal of 80-85% fastballs per game for high school pitchers. I have coached a prospect team the last two summers of 16 year olds, and I would make them throw all fastballs the first time through the order. This was in an effort to make guys locate with movement in the zone to get outs. Also, they would be surprised at the tone they would set for the game by pounding the strike zone early and often.
The point is that yes, you should repeat pitches. I suggest that by not repeating pitches enough, a pitcher becomes predictable. You can throw the same pitch in the same location more than once. You will be surprised at the results you get.