whenever a person go thru the "rotating door" device, he inverted himself, his time is flowing backward, so effectively he's travelling back in time.  So when he go thru the device again, he's back to normal non-inverted flow, but he can't go back to the future where he came from, so effective there will be 2 of him in that timeline until the other go thru the device.

So the Andrei that Kat killed is the future Andrei, and there are a current timeline Andrei somewhere?

So everything has already happened before and effectively they've changed nothing at the end?