To me the theory about Gantz creating both copies and sending the originals is possible. If you consider the kind of technology that Gantz has, memory manipulation is possible. So it is creating a human body from "scratch", since it would be basically transporting matter from different places and simply rearranging it (well, the concept is simple, but I guess the actual process wuoldn't...).
Suposing that's true, then the Kishi incident was just a mistake. If what Gantz brings back to the room is the "fixed" original then the copy that Gantz made of Kishi is because he made a wrong assumption (that she was dead or somthing along those lines). However, since Gantz can recreate the dead and manipulate their memories, it's hard to tell who are real and who are the copies.