[paul.fr]

If i have one room, that is illuminated by a candle and add a mirror behind said candle. Would the room be twice as bright?

[another_someone]

It depends on what was there behind the candle in the first place?  If the surface that was behind the candle was a mat black painted wall, then clearly you have replaced a light absorbing surface with a sight reflecting surface, and so there will be an increase of the amount of light to see by (the exact percentage by which the illumination will increase will depend upon what percentage of the surrounding wall is replaced by a mirror).  If you are replacing a gloss white wall with a mirror, the difference in available light will be marginal.

[lyner]

You can make better use of a candle - for close work, for instance, by using a lens. In the old days, ladies used to use a  large water - filled globe  as a lens  to concentrate more of the candle light to help them with their sewing.  But someone else in the room would be getting less light at the same time.
Chandeliers use the same principle - catching light that would be, otherwise, wasted.