It's not really duplication, imo. It is a common OO practice to create methods that delegate to embedded objects. We could do the base class thing, but the the method names might not match exactly with what we mean them to do (e.g., we'd have `included_items` as opposed to `included_cases`).

It shouldn't lead to much (if any) additional maintenance cost as once the method is in the page object it should remain static. If the implementation changes it will only change in the embedded `MultiselectWidget`, which is the point of the encapsulation.