Just play lightly and everything is going to be OK! series: Repeated Notes
Repeated notes are especially hard for a pianist. To lift one finger up and down again and again requires much more attention from your brain. For a string player, repeated notes are much easier. That is why Suzuki book 1 starts with repeated notes.
Raise your wrist and make a pinching finger position
To play the same note with the same finger is hard. Your muscles become tighter and tighter if you speed up. Usually your wrist should be level with the top of your hand, but to repeat the same note without tightening your muscles, raise your wrist a little bit higher than usual, and gather your finger closer together. This pinched way of playing allows you to relax. Aim for the same spot on a key so you can have more control.
Accompanying pattern VS Melody line
There are many accompanying patterns that include repeated notes. Those accompaniments tend to have more notes than the melody line. So you have to consciously make your playing of the accompaniment pattern softer than the melody. Otherwise the accompaniment will overpower the melody.
Repeated notes and Melody line within one hand
There are passages with repeated notes and a moving melody line at the same time, to be played by one hand. In that case, you have to play the repeated notes very softly and emphasize the melody line.
To practice this technique, I recommend you try Hanon No.6, and Czerny No. 5 and 6 from "125 Exercises in Passage Playing". Combining these two methods is useful, since Hanon is purely finger exercise, that makes your muscles learn; and Czerny is very short, but has important basic chord progressions. Acquaint your brain with the important chord progressions, and then transpose them, and convince your brain!