I came across a question linked here:
[How do they film the mirror scenes in movies?](https://movies.stackexchange.com/questions/2190/how-do-they-film-the-mirror-scenes-in-movies)

I found it interesting that most examples came from popular and from recent films. Here is a clip from Lady in the Lake 1947, the scene shows a man from a POV as he walks through the room (NOTE: the whole film is in first person), the camera turns to the mirror and our hero appears. Now it was probably done with camera angles as it was 1947 but Citizen Kane was 1941 and he implemented techniques beyond his time such as over-lapping shots, blacking out background and foreground and then meshing the shots together. 

My question is can anyone find a reliable source explaining how THIS mirror trick was done in the movie?

The mirror trick starts at 2.04 - the clip is only 2.35