Diplopia is derived from two Greek words – diplo – which means double and -opia – which means vision. The illusion of seeing double comes about when the eye muscles are weakened. When the muscles that control the eye become uncoordinated or just don't work perfectly, you see a double image where there should only be one.
It usually resolves itself over time. Occasionally a short treatment of corticosteroids (or steroid treatment as it is sometimes called), is prescribed to help it along. Your doctor may also have you wear an eye patch on one eye. This isn't a good long-term solution, however. For short tasks, such as driving, it works well.
If you wear an eye patch for too long, it can interfere with the brains ability to produce a single image even with your weakened eye muscles. Some doctors may prescribe special glasses to help with the problem. These glasses have prisms that help minimize the double vision.
Other treatments are eye exercises, surgery, and possibly even botulinum toxin (or botox injections). There are also medications that might help minimize this problem. Oftentimes, when it is the result of an exacerbation, when the exacerbation is over, this symptom goes away as well.
This is just one of the eye symptoms common to multiple sclerosis. Optic neuritis is very common, as are nystagmus. As with double vision, they also resolve themselves over time.