Caius the Mega Monarch if targets 2 cards inflicts damage twice?

Question

Caius the Mega Monarch:

You can Tribute Summon this card by Tributing 1 Tribute Summoned monster. If this card is Tribute Summoned: Target 1 card on the field; banish it, and if you do, inflict 1000 damage to your opponent, then, if it is a DARK Monster Card, banish all cards with that name from the hand, Main Deck, Extra Deck, and GY of the player who controlled it. If this card was Tribute Summoned by Tributing a DARK monster, add this additional effect. ● You can target 2 cards, instead.

If targeted 2 cards, will inflict 1000 or 2000 damage to opponent's LP?

Answer 1

The opponent will take 1000 damage.

Think of it like this; if you tribute a dark monster, you modify the activation cost (i.e., instead of "Target 1 card on the field", it becomes "Target 2 cards on the field;"). The effect simply states "Inflict 1000 damage to your opponent." This means that the modified effect would become,

banish them, and if you do, inflict 1000 damage to your opponent, then, if any of those monsters were DARK Monster Cards, banish all cards with the same name as those DARK Monster Cards from the hand, Main Deck, Extra Deck, and GY of the player who controlled them.

As the 1000 damage is not per card - simply an effect to itself - the amount remains the same.