(29/04/2011 10:43 PM)Assassinator Wrote:  

Spoiler:

My explanation is that there could be only 1 copy of a person in any world at one time.  Madoka isn't really in the "new world" in as a living human being like everyone else.  But she is there, removed from the axis of time, as some sort of omni-present existence that's everywhere at anytime.  So because there is already a Madoka, there cannot be another.