Hmm.... I know in Half-Life you could get that effect by noclipping outside of the map. Basically it doesn't have anything to draw, so it just leaves whatever was previously there. That's how it duplicates it like so. It seems like your skybox just disappeared. Maybe a texture problem. Not sure. Make sure you've got DX9c and the latest drivers for your card. If you do, and it still doesn't work, then try redownloading DoDS.