If the double vision is temporary, it can be caused by tired eyes or too much alcohol. It will disappear by itself several days later. But if it is lasting, it can be several reasons. A head injury, tumor, stroke or related condition can cause double vision that comes on suddenly. Eye disease such as cataract can also cause double vision. The problems in eyes should not be ignored. You'd better see an doctor soon.
Double vision can be caused by problems both in your eyes and your body. Eye probelms such as myopia, astigmatism, dry eyes, cataract and eye injury can cause double vision. Diabetes and cerebral ischemia can also cause it.
Maybe your double vision is caused by eye strain. If you overuse your eyes for a long time, your eyes will get tired and then cause your double vision. It may be caused by some other reasons, so you should go to see an doctor and find out the exact reason.