What can cause sudden double vision?

Diplopia: Double vision can be due to the visual system itself; poor refraction (eye glasses); or may portend serious disease in the intracranial vault and brain (tumors, vascular disease). If you are having this problem, immediately check with your physicians or go to the nearest er for evaluation.

Many causes: Potentially very serious symptom which needs prompt evaluation by an ophthalmologist or neurologist. Possible causes are cranial nerve palsy, tumors, thyroid disease, muscle problems like myesthenia gravis, multiple sclerosis. Less serious causes are possible too.