It is normal for one arm to be stronger than the other. Just do the same number of repetitions for each arm if you want the muscles to be the same in strength and size. Even if you do more repetitions with the left arm it is not going to improve faster than the right arm. It was just stronger to start with. Most likely you use the left muscle more than the right one. When I was doing as many repetitions as possible with both arms I could do more repetitions with my one arm and the muscle was bigger to start with. However they both improved at the same rate.
If the exercise is not enough to balance out your arms then use your right arm more in daily life.