Well other people have the power of the keyblade and they dont look alike. I think that Ven was inside Sora all along and Roxas is the Ven part of Sora that came out when he was turned into a heartless. Now according to the time they gave us Sora was actually alive at the time Ven, Terra, and Aqua were fightig Xehanort. I think that Ven somehow put his being into the keyblade and then chose Sora. Once the nobody was made it was actually Ven's part that came out, which is why Roxas looks exactly like Ven.
I have a couple other theories but I may post them another time
