The case
Disney's Snow White was rereleased in theaters throughout the '70s and '80s and landed on VHS in 1994, so every Gen X kid met the Queen at her mirror. In the film she says "Magic mirror on the wall."
"Mirror, mirror" comes from the older Grimm translations and from the hundreds of parodies that followed the film. The repeated words are more fun to say, so that's the version that stuck.
Why our brains do this
The repeated form is more rhythmic and appears in the source fairy tale, so it competes with, and beats, the film's wording.
Sources: Wikipedia — Snow White and the Seven Dwarfs (1937 film) · Wikipedia — False memory: the Mandela effect
