Who is it that prays more than once yet still doubts that his prayer is valid?

A person who prays more than once and doubts the validity of his prayer is afflicted with waswasa (obsessive whisperings), which is a dangerous disease that corrupts one's religious and worldly life. Its only cure is to turn away from it and ignore it. Therefore, he should perform the prayer once and not repeat it, no matter how much Satan whispers to him that it is invalid.