/vr/ - Retro Games

Duke Nukem 3D does NOT have an "identical room on the other side of the mirror manually built by the level designer". That's just bullshit misconception made up by people who've never opened Build in their life. The mirror is handled by game code, all the level designer needs is to add one empty sector behind the mirror. It's one of the common misconceptions about Duke and Build that is very often heard, like people who claim it's "raytracing" or people who have no idea what portals are.
Pic related, E1L1 mirror and empty sector on the other side. Not that requiring such a huge sector behind the mirror was only true in version 1.3, in Atomic Edition even that is not required anymore.

Only one of the console ports of the game used the "identical room manually built" thing, I can't remember if it was the PSX or the N64 version.