If you delete some question sessions, the results of the overall statistics will stay the same. Only the session itself (visible in the overview) will be deleted permanently. Your overall statistics include every question, regardless of how many times you have answered it.
Your Learning recommendations, as well as your evaluation by Organ Systems / Discipline / Learning Cards will only include every question once - but you can always choose between your Initial or your Latest answer profile.
To change this option, go to Analysis > Study Summary and choose the option you wish under the Performance totals: