If I have a hero with a Magic Blast (Ranged Damage) and I add Magic Blast twice to the list, every rank in the skill counts twice to the bonus. If I make a selection and then add another attack but don't fill it in, that undesignated attack maps to the last item picked (here, the Magic Blast) and it again acts as double. If there's more than one entry, the undesignated row only applies to the most recent one.