This story isn’t bad but would be better fit for a play. Kiyoshi Kurosawa, director, doesn’t seem to care about continuity errors: the film is full of them. Indeed, people don’t react to Hayasaki’s interactions with his double. A film of this kind would usually allow the audience to draw their own conclusions. Sadly, when every thing is said and done, you will have learned nothing. You will not be able to seek refuge in supernatural conclusions nor in concrete reasons.
From a more positive perspective, it would be a shame to discredit Doppelganger simply because its premise doesn’t announce anything new. The film offers two versions of its main character when in fact it could have shown two divergent personalities of the same individual.
Doppelganger starts making sense in its final punch, which I greatly appreciated. The secondary intrigue, at first glance, is brilliantly merged with the main one, leaving us on an interesting note.