I got to point out Gantz didn't just "clone" kurouno at that time. We SEE him being teleported from OUTSIDE to INSIDE. There was no outer body for this scenario to happen to. Everyone outside saw Kurouno being teleported back in(well at least those who were looking at him at the time). That phenomena has been explained before, gantz reloads the person before they recieve damage, and by far thats the most reasonable explination. Further proof that there was no random body left behind, check the end of the dino mission where the vampires are first introduced, they start fucking up who is left over and when said people teleport they dont fight some copy that was left behind because they are GONE, as in not there.
Now until they actually show a flashback showing otherwise, this "theory" has little to no merits within.