Another change

"... I wish that for the next ten minutes, Jon will be an exact duplicate of myself." The rock grew warm in Karyn's hand, and now standing in Jon's place was another Karyn. She looked confused. "What the... Why am I at Jon's house?" the new Karyn asked. "Jon, are you okay?" the original Karyn asked. "Wait a second... you're me. How did that happen?" the former Jon spotted the rock, "That's it. You're Jon, aren't you. You must have changed yourself then somehow wished me here. Well, you'd better change back." Original Karyn just stared at new Karyn. "No, you're Jon, and I changed you into me." "That's ridiculous. I think I'd remember who I used to be. Now give me the rock Jon." But she wouldn't, and soon a scuffle began between the two Karyns. Who would get the rock?