In watching this episode in the past, it never has struck me before that Josef seems to know a lot more than he is letting on but this time through, I had a much stronger impression of this.
The statement that, 'everyone has a double' seems on the surface to be very cut and dried, but I would think you would at least get a little bit more information before you assume that you know how things are. Especially when you consider that these are vampires we are talking about.