I don't know, this is really weird. But as we can remember Kurono has died twice (the first one at the very beginning of the manga and after the vampires killed him), it has kind of logic the fact that it can be two of him, yet when he had a "second chance" he came up as a gantzer.

Now about Gantz walking like nothing in the city, there must be a reason. I just hope that this won't end in a meeting with another gantz-ball guys -__-.