Is it unhealthy if your index finger can bend all the way back and touch the back of ones hand?

As long as your not forcing your finger back and it is causing no pain then it's absolutley normal for your hand. Some people have hyperextended or doublejointed joints. This basically means that joint can extend the extent of it's bending ability or can twist and turn in weird yet wonderful ways. There's nothing to worry about. However, if it is causing you pain then i suggest go to the doctors to get it checked out but i shouldn't worry if i was you, it's a talent, not many people you know will be able to do that. SURPRISE THEM WITH IT!