Obviously, because you didn't 'give you the book', you made an (illegal) copy and kept the book too.
It happens we do have a True Duplicator: the internet. It only works on digital goods, but it already matters a lot. The lack of exclusivity makes anyone lucky enough to be connected wealthier, at very little cost. Again, how can we justify the exclusivity?
My conclusion is that the illegality of the copies only proves the immorality of the law. We ought to find better solutions.