if it true that you can have two ponies with same cutie mark it seem there poines in this shows with the same cutie mark if not then this blue blond hair pónei, pônei is doctor whooves or it was just a error the animeator made.this is a real clip from the ep at the gala
During the song, fluttershy face this pónei, pônei with an hora glass as a cutie mark,doctor whooves has the same mark.
also there are other ponies there with the same cutie mark like the the prince that raity wanted to be with and the two colts blue and gray one that are welcoming raity when she about to sing her part in the song,or it just they got to lazy to draw new cutie marks for them
or there a chance that siblings can have the same cutie mark.........just kidding i dont know but maybe