Why two return statements only get one output not two?

Hey, mates! I coded as follows, in the body of defining “double_type” method,I wrote two return statements, I assumed there would be two outputs when calling the method, however, when I acturally called it, there was just one output presented, it confused me, could anybody clarify where I made a mistake? Thank you!

Because anything after ‘return’ is not reachable.