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?
Through the years, it’s been a common misconception that being double jointed is a bad thing. Actually, there is no such thing as a “double joint”. While some people can bend their thumb’s back to touch their forearm, it may appear that they are “double jointed, but there is only one joint at each joint, not two.