Is it normal to have double vision while writing or reading or watching tv for long in dark?

It can be.: Many people will have some degree of misalignment between their two eyes when fatigued. This is called a phoria. It is normal. Patients will experience double vision temporarily, but should easily be able to eliminate the double vision by "refocusing". Double vision that does not go away or happens frequently is abnormal and should be evaluated by an eye doctor.

No: Double vision is never "normal". There can be some serious causes of double vision, so it's worth getting checked by an eye md. In your case it sounds like its related to fatigue.