Oh that is bizzare!
Thats not actually the doorway itself duplicating, whats happened there is everything beyond the doorway is not even loaded and so its what is called the void (i believe thats right), its the space outside of a game map/area, because there is literally nothing there the effect that is produced is that everything that is loaded (so that door frame there) and is within your view, is perceived to be duplicating when its really not, its just an effect.
Im sure someone could explain it better, but this is the best i got (plus i used to make maps for a Half-Life mod called Sven Co-op and also Garry's Mod and this similar thing happens there if your map has a 'leak' (basically a gap into the void)
I know its not the answer you were looking for though 
Best bet i would say is to actually restart if you can
