Identifying new references when appending to existing reference list
If you are using the EndNote reference manager, it will tell you that you are importing duplicates, which references are duplicates, and then give you options for what you can do with them. It seems reasonable that other reference managers will do the same.

As an example, many journals now have adopted the practice of publishing articles online before they have been assigned to a print volume. Thus, I may want the manager to replace the old reference (without page and volume information) with the new (with that information). EndNote lets me choose to do this.