Question: If a person can hit a forehand shot with either the right or left hand is it necessary to also learn the backhand?

No, it is a waste of time. But you have to know which one of your two hands is more effective. If you know the right hand is more effective, then you have to play with the right hand and use the left hand only for emergencies. You may think they are equal, but one hand is always stronger, and the stronger one you have to make the strongest. That does not mean you should neglect the other hand. You have to try to make it better, but this may take a longer time. So the one that is better you should make best, and the one that is good you can make better.