Instances of TypeMirror that are equal (TypeMirror::equals), aren't necessarily same (Types::isSameType). If care is not taken when putting instances of TypeMirror into a set, that set might end up containing same instances.

If Utils.getAllInterfaces is called on a type that extends or implements a particular interface multiple times (on different levels of its hierarchy), the returned set might contain multiple representations of that interface. For example, I've seen a case where getAllInterfaces that was passed a TypeElement corresponding to java.util.ArrayList returned a set containing 3 instances of TypeMirror corresponding to java.util.Collection<E>.