Through the years, it's been a common misconception that being double jointed is a bad thing. When 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.
Here's what's really going on. What we call "double jointed" is really just more than usual flexibility around a joint that results in a greater range of motion. In other words, the joint is too flexible. It's common in children, and tends to be lost as we age.
Too much flexibility can result is joint instability. In other words, the joint might move so much beyond what is normal that it can result in a dislocation, sprain, or ligament damage. If you are "double jointed" no need to worry, just be careful. You can do exercises to strengthen the muscles around the "double joints" and this can help reduce the chance of injury.