Yeah, movies could be nominated for more than one category if they fit in more than one. A few movies are like that (Robocop, Lawrance of Arabia, The Dark Knight) ... It's because we're deciding the best movie from each category as subprizes, and then the best overall movie as the main prize. If a movie wins in two categories, it will go up against itself in round five.
Where would you put Ghostbusters, in horror? I think it's more of a comedy than anything else.
Sci-Fi? Just compared to the films around it in comedy, its hardly comedy.