Well, first, I have to tell you that there are many reasons which can cause the double vision, such as alcohol, stress, and injury to the eyes. So for your father's situation, it may caused by alcohol. Maybe you can check it with your father. And as we know that double vision is a condition in which, whenever you look at any object, you see its two images rather than one. And according to some experts, it is totally different from blurred vision, in which you see a single blurry and unclear image of an object. For the side effects, you should know that when suffering from double vision, it will severely harm decision-making capacity and quite often. And what is more, if you insist on driving, put others as well as yours life at risk. So just be careful about it.