Yeah... You was right... thanks for the replay

" The Problem that you have is that there are always 2 faces on top of each other.... that's something what you can't really see.... and it's a little bit weird cause the vertex count is right. You can check that out if you select 1 face and delete it... you will see that even after deleting it there's still a face at that spot. That means you have twice as much faces in that scene as you should have.

Well, that might have happend when you duplicated that mesh... I guess you have duplicated it 1 time too much and accidentally combined 2 meshes that were on the exact same spot in that scene. But that's only a guess :-)

All I did was selecting your Object and then under "Mesh" ----> "cleanup" I checked the options as you can see on the screenshot I attached to this mail. What that did was it selected all the faces wich are "zero geometry"... all I did then was hit delete on the keyboard.... and that fixed your problem."