What is the ruling on a person who repeats his prayer because he doubts his prayer was correct?

What is the ruling on a person who repeats his prayers on account of doubts? The prophet said to not pray the same prayer twice in one day. So, if after a person has prayed in congregation he prays at home again because he believes that his prayer was incorrect or does not know the ruling on a particular mistake, which prayer will be accepted of him? Or are both correct?