My Theory of the Movie Plot
When I first watched this movie I was- like the rest of you- pretty disapointed with the ending. It seemed like nothing was explained, it didn't make any sense. I decided to go back and watch it meticulously to see if I had missed anything. I now have a theory of what the heck was going on.
When the movie begins with the birthday party noone was a duplicate. I believe the first smashed mirror let loose the alternate of the father. We're not supposed to understand how this is significant. However it happened in his home just like the appearances of the other duplicates happened in their homes. It's hard to connect the dots because the father isn't replaced until much later in the movie.
The daughter returns home and when she goes back to work her duplicate emerges from the smashed mirror. Next we see the daughter at work and becoming aware that her co-worker saw her duplicate. The duplicate had followed her to work and then left for no apparent reason.
The next scene shows the daughter in a phonebooth- I believe that from this point on we are following the role of the duplicate NOT the real daughter. She is calling her "boyfriend" but knows that he too has been replaced. When she sees the real daughter drive by she follows her to make her attack. If you watch the following scenes with this in mind- it can be interpreted that the duplicate is not actually scared but pursuing her victum. Also there is only one car and the duplicate steals it after she eliminates the real daughter. (Notice the liscense plate is a mirror image- Y38 BCY).
Next the duplicate is in a car accident. This leads to her loss of memory and essentially forgetting that it is her who is the duplicate. She now thinks that she is the REAL daughter.
To make a long story short (shorter)- the rest of the movie is all about the duplicate regaining her memory until the final moment when she gains total recall. At this point she remembers everything and realizes she is the duplicate. Her father has also been replaced at this point and she knows that too.
So that's my theory. Watch the movie with this in mind and it may make sense to you. There are many other clues throughout but I think I've already written enough! cheers