double, or a superimposed "ghost image" of an object through one eye is called Monocular
Diplopia. The most common cause is a developing cataract.
"ghost image" is caused by a different focusing ("refractive") power of the
different layers of the natural human crystalline lens. Rather than being solid
as you might think any lens would be, the human natural lens is built like an
onion, in layers, growing outwards like the growth rings of a tree. If the
central part of the lens focuses a little differently than the next layer due
to cataract, you will see double out of one eye.
(and if) the monocular diplopia becomes bad enough, you may need to have a
cataract/lens implant procedure.
_Written by J. Trevor Woodhams, M.D. - Chief
of Surgery, Woodhams Eye Clinic