Does the reviewer ever get the same manuscript twice? Consider the scenario where an author sends his paper to Journal A. Journal A then sends the paper to reviewer X and the reviewer rejects the papers. After rejection, the author send his paper to Journal B. Journal B then sends this paper to reviewer X. 

Does this ever happen?
What does the reviewer do in this case?
It happens all the time. I think it is the reviewer's responsibility to respond to the editor saying that he or she has already seen the paper. Then it's the editor's call. Ideally the paper would be sent to a new reviewer, but depending on the subject of the paper the pool of available referees might be small enough that the editor requests the reviewer to reread the paper anyway.