We've had quite few pairs of twins in K-pop industry, but never an identical one! f(x)'s dancing queen Luna revealed that she has an identical and biological twin sister, by posting a photo of both posing in front of camera to celebrate their birthday. Both Luna and her sister turned 17 on August 12th. Including myself, netizens had quite a hard time differentiating the two in the photo. Frankly, there's simply no way to tell! Which one is Luna, do you think? The sporty one, or the lady-like one?