Items that already exist in the collection have a different color in the result list when you scan them and if you proceed to add them you are given the choice to ‘CANCEL’ or ‘ADD AS NEW’. This is the desired behaviour but today I have noticed that this doesn’t apply to box sets. Existing box sets have no different color and are added regardless without giving the user a choice to cancel the add.
If you add as multiple entries then indeed it won’t show as “in collection” as that comparison is done based on “single movie entries”. It is indeed a bug, and I have it on our todo list.