this is usually due to convergence insufficiency. do convergence excercise process. take a pointed object out it 2 feet away from your eyes and slowly bring it closer. at the same time concentrate on a distant bar object like a vertical rod. when you wil concentrate on both things you will observe that Rods are two and the pointed object is one. but as you bring this pointed object nearer you will see both the objects have gone double at this point squeeze your eyes forcibly for 30 seconds. repeat it for 10 to 20 times a day for one month.