Visited eyes specialist as well neurologist but had few test too but they still not able to predict what is the cuse of diplopia.

Not always: In many cases there is not any identified etiology for diplopia which is produced but the asymmetric movements of the both eyes.... After ruling out problems with the cranial nerves, intracranial structures, abnormalities in the union between the nerve and the muscle [such as myasthenia gravis, botulism, etc]... Exercises and/or surgical local correction may help....

Yes: But there are multiple causes and need it work through common to uncommon to decipher the puzzle.And sometimes takes time to have cause clarify itself!