Seeing two images where there should be one, particularly when it starts without warning, may indicate a problem with the nerves or muscles that control eye movement, or a problem originating in the brain. When sudden double vision is accompanied by headache, weakness, or changes in speech, it may signal a stroke and requires an immediate 911 call.
You can gather useful information by covering one eye at a time. If the doubling disappears when one eye is covered, it is likely binocular and involves eye alignment or nerve function. If it persists with one eye covered, it is monocular and may relate to the eye itself. Share this observation with your eye care provider, as it helps narrow the diagnosis quickly.