neurological probs can often create double vision and it can also be intermittent ie. worse some days than others. Muscle weakness of one of the six muscles controlling eye position will cause double vision and it will always correct itself if one eye is covered or closed if muscle weakness is the cause. your optometrist may be able to fit a prism into a pair of specs which can alleviate symptoms. If it's intermittent a Fresnel prism (which is removeable) can be fitted to your specs, but this can make your vision worse (less clear but not double).The Fresnel prism will avoid thick chunky spec lenses as well.

If your a contacts wearer and get double vision in one eye with the other eye closed or covered, it could simply be a badly fitting contact lens, a simple check would be remove your contacts - does the problem go away when specs are put on? In either case you should still see your optometrist so they can rule out any other problems. Hope this info helps.