This is further strengthened by the other actor having some kind of green contraption attached to his back, which the person behind FlashFilmNews speculated could be used as a lighting reference to replace this actor’s face with Ezra Miller’s in postproduction.
(Remember when Ezra’s Barry ran into Grant Gustin’s version of the Fastest Man Alive?).
However, perhaps director Andy Muschietti and the rest of the creative minds decided it would be better if for this movie, any other Barry Allen introduced should look just like Miller.
Now given the fact that Michael Keaton and Ben Affleck’s respective Batmen are both in The Flash, not to mention what we’ve seen in the Arrowverse starting with Crisis on Infinite Earths, not all doppelgängers in the live-action DC multiverse need to look exactly alike.