It may be a sign of myopia if you always suffered this. However, it's pretty easy for people to get double vision when they get tired. A person with myopia can see near objects clearly but has difficulty seeing distant objects. You need to check out how long you need to get back from blurred vision. You'd probably get myopia in the near future if you need a long time.
But there needn't to worry, because you can do some remedies to bring it back. Take a break once a hour to see some green plaints, then massage your eyes for a while.
Well, yes, it is normal to have double vision when you read book for a long time. According to some researches, if you read book for a long time, it will make your eyes focus on the small points too long. In that way, it will just increase the eye strain, making your eyes stressed. In some cases, double vision can occur. On the other hand, for myopia, it will be hard for you to read at a long distance, even if you are very young. But from your situation, it is hard to say that it is a sign of myopia. So you need to console it with your eye doctor.
That depends. How long the distance when you read? What view were you looked for? If you red a book in your hand within 5 centimeters, and find the double vision appeared, it may not only be the myopia except the strong prescription. Myopia's feature is vision clear in short distance but blur in far distance. If you read in near distance with double vision, it may be the astigmatism. It is a kind of optics defect in an optical system in which light rays from a single point fail to converge in a single focal point. What the hell of your eyes disease, you would better go to hospital and use professional instruments for check!