Merle (dapple) genetics are absolutely fascinating! I've spent a fair amount of time researching doubles, and the merle and harle gene because of it.
I think the most important thing to remember about merle is that it's dominant, both parents MUST me merle, in order fot there to be a risk of a double dapple. Though cryptic (hidden) merles/dapples are out there, there's usually a way to tell (a small spot, or looked dapple as a pup then it faded, etc) but not always. Some colours wont show the dapple. which is one of the reasons there are so many double dapple dachshunds out there that only have cream or yellow markings but no actual dappling.