Use HashSet or flags for choices

[Arithmomaniac]

Right now, a choice list is a List<T>. This doesn't ensure that the selected choices are unique, and could lead to strange behavior and/or failed API calls if a choice is added to the list multiple times.

We should use either flag enums instead (and then the choice list can just be of type T), or the choice list should be a HashSet<T> instead.

I favor using HashSet.

[Arithmomaniac]

We could also just ensure people could use whatever ICollection<T> they want - which would not be a breaking change after all.