Double vision can be a sign of visual fatigue or an incorrect eye glass prescription. As you suffer from this now and then, I think your problem may be caused by fatigue. Are you lack of sleep or work too hard recently? You can have a good rest to solve your problem. Double vision can be called diplopia. But sometimes it can be a symptom of serious illness, such as blood clot, tumor, trauma etc.
Double vision or diplopia can be binocular and monocular. Neurological diseases and inflammation can cause double vision. Some diseases such as thyroid conditions, meningitis, multiple sclerosis, strokes may cause double vision. I know that cranial nerve damage and thyroid conditions are the two most common causes of double vision in adults who is older than 50.But you are only 30 years old. So I am not sure. You can consult your doctor and have your eyes examined.