I don't think there is anything called a double comb. There is a comb called a buttercup that is "double". There are also rose combs, pea combs, walnut combs, modified combs and a few others that I have forgotten. There is a chart that shows the different shaped combs but I don't have it to post for you. Maybe someone who has it will post it. Different breeds have different combs.
I have one just like your 2 blue ones....she will be an olive egger, which falls into the easter egger category, TSC sells them as Americaunas, though they are not true Americaunas, they are mixed breeds. So my guess is an easter egger, they had some little blue ones at my TSC in the Americauna bin. But they also had silkies with yellow legs and beaks, which are probably mixed breeds to.
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suze, have you looked into what you get with the "Blue Gene"? You can get three possibilities: Black, Blue (varying shades of gray with usually darker feathering around the neck (hackles) and on the males also in the tail and the saddle feathers), the other is Splash with is any variation of white with black or gray spotting/speckling in no real pattern. Depending what color you breed to what, you can predict you odds on what colors you will get. I am sure there is a thread here that explains it in detail if you need to know more.