Mark,
I don't recall having seen a double diopter before this one.I don't know how common they are, I just didn't pay much attention to them before. There is a single one on the 10x42R (Val Kern)I just got, but Since I haven't started loading for it yet, I haven't tried it yet. Of course, I have often used American style peep sights, but I understand the German style is used differently.I understand you still use the rear sight, but through the diopter( to sharpen the sight picture). It seemed to help, but I havent actually fired it yet.
Mike