Thank you for answering
I know in Maya it works fine, but actually I'm trying to make a Sims 2 (a game, in case you don't know) object. The object showed in the picture is extracted from the game. Here is picture that shows what the problem will be if a plane has only one normal (left is in game effect and right is in maya). You can see the stamens not display correctly, because the normals of petals is pointing into the middle of the flower. However, if I simply reverse the normals, the petals on far side won't show because they are pointing away from the camera. Thus in order to make it work, I need to have normals on both side of the plan, as the picture above