no such thing
Well, my horse sometimes does this too, rests his hind legs in all sorts of funny positions, but it does not cause any issues while riding.
Having a horse that is toed in, toed out, can sometimes cause problems while riding, especially if they paddle, not to mention that it is unsightly.
But to answer your question: no it it not possible for horses or humans for that matter to be double jointed! That is a phrase very commonly mis used.
Your horse is just special and very flexible! This condition, which is the same in humans, is called hypermobility. This means that the joint, along with the cartilage and ligaments are just super flexible. Alot of the times it is genetic and therefore passed down by parents.
The reason to why it is impossible to be "double jointed" is because a joint is where two bones meet, and it would be impossible to move it both ways equally without causing dislocation.
This being said, a human or horse with this condition might have more chance of a dislocation, but it is nothing to be worried about or to change a riding schedual over.
Also, the flexibility will gradually lessen over time, as the horse (or human!) gets older and more conditioned.