Actually, all lenses when stopped down will produce twice as many diffraction spikes as there are blades. Unless they have very rounded aperture blades, that is...but that's a different discussion.
Each straight edge in the iris is responsible for creating a pair of spikes. But in the case of an even number of blades, the spikes from opposite sides line up. Think of a hexagon, and how each face has a parallel opposite face, whereas a pentagon does not have that property.
So a 6-bladed aperture does, indeed, create 12 diffraction spikes, just as a 7-bladed aperture creates 14. But half of the 12 spikes caused by the hexagon lie on top of a twin, and therefore there appear to be only six. The can be evident if the two opposing blades aren't perfectly parallel. Look at the first picture in this thread. The spikes at 1 o'clock and 7 o'clock are noticeably doubled. That's because the edges forming those spikes aren't exactly parallel, and the resultant spikes are just a tiny bit misaligned. All six spikes in that image are actually made up of two...it's just that one pair that reveals the secret doubling.
14 pointed star . Ha
Try compare between 2 shutter speeds, 1 more slower than the other and see the starburst effect..
