There are many causes of double vision. Some are more serious than others.
If double vision goes away when either eye is covered, the eyes are misaligned. This could be due to a problem with the muscles that move the eyeball or with neurological control of these muscles. One of the most common causes is loss of control of a longstanding misalignment. Another common cause is a mini-stroke that affects only the muscle that controls the eye movement. Assessment by a GP, as well as an ophthalmologist, is required to minimize the risk of further strokes.
If the double vision or ghosting persists when the other eye is covered, the problem is usually structural. This could be due to corneal astigmatism, lens opacity, or an epiretinal membrane.
There are other, less common, causes of double vision.
Everyone who experiences double vision should have their eyes examined without delay as some causes of double vision may be life- or vision-threatening.