The Demi Moore film “The Substance,” currently in theaters, has a doppelganger in its midst: a dark comedy likewise about an actor undergoing a radical exterior transformation while overlooking some nagging problems under the skin. It’s called “A Different Man,” starring Sebastian Stan, and writer-director Aaron Schimberg explores ego, self-esteem, envy and desire in different and craftier ways than anything in “The Substance.” With a crucial performance from Adam Pearson to complement Stan’s fine work, the film is well worth seeing.