Double Attack EXPLAINED (MUST WATCH Before OP-06) | One Piece Card Game 

en.onepiece-cardgame.com/rules/

Like explained in the video, it's a confusion that players have from other TCG. In One Piece Card Game Double Attack applies only on attack that connects on opponent's Leader as it's the only instance of "doing damage" and instead of 1 damage = 1 life card, we deal 2 damage = opponent must take 2 life cards.

Yes, any trigger effect must be resolved first, if activated, before taking the 2nd damage from double attack.