But the Devil Fruits...didn't Oda once say that Devil Fruits can't exist in two places at the same time? That the user has to die for that Devil Fruit to pop up again?
Well, according to the lastest chapter, there's an unlimited power of Brook's soul that keeps him in this world. As we saw on the chapter, we can clearly take off any of his bones and scatter them across the world if we wanted to. Brook's soul would still be powerful enough to stay in this world. In other words: he can't die!
All of this came from his Devil Fruit. If he never ate it, his soul would have been sent to wherever the souls go to...the Soul Society. This fruit now gave Brook the power to stay in this world, and never die after his first death.
Now here's where my theory is in: If Brook can't die ever again, does this mean he's the only eater of the Revive Revive Fruit? Because if Devil Fruits can't appear again until the user is eliminated, then whoever ate the fruit cannot be eliminated, because it lets their souls become so powerful it cannot let the user die! If somebody ate the fruit before Brook, they'd still have it because they couldn't die. So does this mean Brook is the first, and last, eater of this Devil Fruit? A Devil Fruit that will never again be reborn because its user cannot be eliminated?