The definition of that would be "mono-ocular diplopia" or even more accurate mono-ocular ghosting. Common causes are needing glasses especially for astigmatism or improper glasses or contacts, cataracts, cornea or macular disease. You would need to see an Eye MD ophthalmologist, have a complete exam that included cornea topography and macular OCT testing. Thyroid disease can cause true double vision but not at all common for monocular ghosting