What does "if this Digimon's stack has 2 or more same-level cards" mean, exactly?
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It refers to when there are 2 or more cards with the same level among all of the stacked cards. Examples include when a level 5 Digimon has a level 5 digivolution card or when a level 6 Digimon has 2 level 5 digivolution cards.
Source: BT22-102 Sayo