I can think of several ways. An obvious one is a pair of identical twin models, with the “mirror” being transparent glass. Another is blue/green screen. A third is essentially the same as the second with the selection of the mirror done by hand in Photoshop, simplified by most of the mask being the straight lines of the mirror frame.
There’s a small smudge in the upper left-hand corner of the mirror that adds to the realism. Perhaps it is actually there.