Rules FAQ
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You can choose to sacrifice the character you just put into play.
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If the character enters play as a duplicate, the post-then part of the effect will not resolve, so you do not need to sacrifice anything.
Reaction: After you win a challenge as the defending player, search the top 10 cards of your deck for a character and put it into play. Shuffle your deck. Then, sacrifice a character that shares a Trait with that character.
Link: Decklists
Rules FAQ
You can choose to sacrifice the character you just put into play.
If the character enters play as a duplicate, the post-then part of the effect will not resolve, so you do not need to sacrifice anything.