MoveIt ! Rviz plugin show mutiple pr2 model in disorder when I fellow the quick start tutorial

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I try to follow the quick start tutorial of the MoveIt. When I use rviz plugin to show the pr2 model and try to interact with the pr2. The tutorial can be found at MoveIt webside. but the problem is that when I launch the demo.launch file.The rviz show pr2 in disorder (sometime pr2 is lying on the ground,sometime pr2 is hanging in the air),and the pr2 model dynamicly change in the rviz .

what is the problem?who can help me?