When adding an Edition to a CustomList (as when importing the NYT best-seller list), it's important to avoid duplicate entries. This can happen if a book shows up with slightly different title or author strings week-over-week. This was the original use case for CustomList, so the code was written with this case in mind.--

However, there's another use case for CustomList: using the admin interface to add specific Works to the list. This is now the dominant use case for CustomList, and here the user may reasonably want to add seemingly "duplicate" entries to a list.

Basically, if you add an Edition to a CustomList, a duplicate check for other Works using that Edition or similar Editions is fine. If you add a Work to a CustomList, the only duplicate check should be checking whether that specific Work is already on the list.