Why? -- Stephan's dog; Stephan's leaking ceiling
When (mirror-)Stephan brings (mirror-)Gina to his flat after the accident, the dog (I presume it belongs to real-Stephan) only greets her, not him. Later it even bites (mirror-)Stephan's hand, clearly recognizing that this is not its real owner.
However, it's also not the real Gina there, yet the dog quite happily accepts her, when, really, it should've been equally hostile towards both of them.
The other thing that really bugged me was the water dripping from the ceiling in Stephan's bathroom. The water is leaking from a broken(?) pipe upstairs -- which just happens to be where mirror-Stephan stashed real-Stephan's body, because otherwise mirror-Gina in her amnesiac state wouldn't have found said body.. how convenient....!
Or maybe we were supposed to infer that the broken pipe is a direct result of how real-Stephan was murdered? I guess the scene that would have clarified this may have been cut -- in favor of some more slow-mo's of the car crash...
Tricking the audience is a pretty cheap move. And I say this even though I sort of liked the movie all in all.
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I don't know, Butchie, instead.