The Prophecy: that none can kill Voldy except for the one in the prophecy. IF you see it the other way.. none can kill Voldy b/c he's powerful and that DD might be the only one that can killed him, but DD says so himself that killing Voldy does not satisfy him and there are things that are worse than death.... And Harry alone might be able to do it cuz of probably revenge(opinion) for his parents and he in the future is powerful enough to duel Voldy equally. In GoF, Harry first time dueling Voldy but he's still too weak to dueling him equally. In his coming future, he will find that new power inside him and Voldy will try to kill him before that happens.
Harry can be killed by other causes and Voldy can be killed by DD.
Many people have thought the same way that Harry can't be killed by others nor can Voldy.. But if you think about it. It means the other way.Prophecy like this i have read before in many fantasy books...
WEll anyway i might be wrong as well
. Hope we can ask JK about it