Hard to believe this is the first time.....
*Spoilers* of course.....
It's hard for me to believe that this is the first time that something went wrong. I suppose we technically can't be sure whether or not previous couples retreat did not go as planned,but Sophia 2 seemed to imply that this was an unusual case of something going wrong. I'm sure I'm over analyzing, but there has to be thousands of possibilities in such a scenario, which would/could lead to thousands of unintended consequences.
Had this been my wife and I, (I highly doubt she would have been fooled by my doppelganger), and we were sitting in the diner planning our next move as they were midway through the movie, we would have either A. gotten the police involved or B. went home and gone after the Doctor.
However, think of all the other possibilities of how this could have gone wrong: Murder, threesome sexual activity, foursome sexual activity, all four getting along and trying to leave simultaneously, further confusion of who is who, etc.
I guess I just find it impossible believe that this would work successfully at all. I did, however, enjoy this movie.