This was such an amazing and unique idea for a movie. It's ashame that the story was ruined by bad writing, continuity issues and too many plot holes. How did this get any positive reviews?
When the dopplegangers first showed up, why didn't they put up more of a fight? They didn't have any guns, just scissors. The husband basically let himself be dragged away. Also, the cops were on their way and said they would be there in 15 minutes, but never showed up!!
At first the dopplegangers appear to be strong and intelligent, but when the family finally fought back, they killed them with ease, and the dopplegangers were dumb - the little boy doppleganger walked backwards into a fire...genius.
Lastly, the plot twist at the end, showing what really happened when the mother met her doppleganger when she was a little girl, and the doppleganger attacked her and took her place. So the doppleganger had been living her entire life...absurd! It contradicts so many previous parts of the movie.
Anyway, it's too bad because it really was a great concept and could have been a great story.