Diplopia is more commonly known as "double vision" because that is exactly what you see, double everything! This is usually (but not always) caused by a lesion in the brain stem near the cranial nerves that control various eye muscles.
How to Treat Diplopia
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What I did to allow light into my eye (while still blocking out visual information that contributed to my double vision) is I simply put a thin medical tape over one lens of my glasses. This tape was light and thin enough to let light through to my eye while keeping me from actually seeing anything with that eye. I still switched the tape from one lens to the other but not as often since it was a tad tricky to peel the tape up and re-tape to the opposite lens every time.
I have had double vision many times since the start of my MS but to be honest, it really didn't start happening much at first, mostly not for a while after my actual diagnosis. I remember it going like this; I would wake up, the ceiling (which is all white) looked normal like any other morning, then I would sit up and notice things don't look quite right in the room, once I stood, woke up, rubbed my eyes, and started walking around it would dawn on me, "I can't really see what I am doing". Like a camera slowly coming into focus my eyes would adjust but never into a crystal clear image, everything was kind of blurry, blurry but different, then I would realize, I see two of everything!