This list has been around and in my possession for few years. It's also considered one of the extras, like the map and the orbs.
Recently I got to thinking about the chapters that we've been introduced to various Duets. With the conclusion to Part 3, I asked Jamie about posting the list and he agreed with it. Some of them have been mentioned several time while some only once, and some not at all. At least now, we know who they all are.
Only Jamie knows if we will see more of them in the future.