Double-jointedness is a myth. Though you might be able to stretch your muscles twice as far as others, you owe it to a single, extra-flexible joint.
Can you bend your thumb backwards until it touches your wrist? If you join your hands behind your back, can you lift them over your head without letting go? If you can, you might be what some people call “double-jointed.” But, A Moment of Science wonders, how could anyone have double joints?