Courses > Reading lists : On the “Edit reading list” screen, it should be possible to see the Item call number too (and not only the Call number). In our library we sometimes have several copies of a document which means that we have multiple items in one holding.
In situation A, only one of the items is added to a Reading list.
In situation B, different items are added to different Reading lists. But since you don't see the Item call number on the "Edit reading list" screen, it is not obvious which item is in which Reading list.