Maybe there’s a cultural idea about mirrors being somehow “the same”. After all, a mirror shows the same thing regardless of which one it is. Or related in cultural mythology to a singular adjoining world that contains your doppelganger (in such media, you don’t usually have a separate mirror-self for every mirror, but one that can be accessed from any mirror). Also could be a turn of phrase that stuck without a good reason.
Pretty confident in my solve. The only ones I didn’t get myself were 20-down, 29-down (obvious in retrospect), and 21-across (inferred the word, but didn’t know the tool).
spoiler
N I X O S O P K C P G T K O E M A C S R N D E B I A N L C L I O N E T S I R N M H U M I N T E D I T O R S P H O E L E M E N T A R Y N U C L E I A I P L G S N G E N D E R L S U L G L E X F C E A L S P E L N S Z O R I N O S C D P T