And you thought the "double rainbow" was cool...no, there hasn't been a second sun hiding out there somewhere, and no, China is definitely not Tatooine! The sight of two setting Suns was sadly just a mirage. This particular optical refraction is not able to be fully explained, however, because mirages usually are visible above or below the original object and not beside it as shown in the Chinese photos.
The other amazing characteristic of this sighting is that the second Sun appears to be the same size as the original, which is abnormal for most types of refraction. This sort of phenomenon is quite rare, but typically is visible near the horizon, where the air is thicker and more readily bends sunlight.
Check out this article form MSNBC for a more thorough explanation of why this happened.