My Little Pony Equestria Girls- Seeing Double

A rogue changeling has stolen a powerful artifact from Princess Twilight, and discovers the portal to Canterlot High while fleeing from the Mane 6. Twilight warns Sunset Shimmer, only to learn that a girl calling herself Twilight Sparkle has already appeared at Canterlot High. Twilight decides to return to the school to face this 'imposter', but is this girl really the changeling?