It punishes the opponent for trying to overextend with multiples of the same card. A particular example of this in the worlds meta was Spell Suppression and Everlasting Cold.
EDIT: Additionally, as you are playing solo, you generally won't have 2 cards out with the same name as they would occupy the same equipment slot, so it rarely affects you.

Level 0 Judge (Because apparently under 16 year olds can't get Judge certificates =/)
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