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Two wizards hold an action to cast fireball on the first humanoid that walks through a door. A humanoid walks through the door.
Combining Game Effects (p. 252). This is a new subsection at the end of the “Combat” section:
Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. For example, if a target is ignited by a fire elemental’s Fire Form trait, the ongoing fire damage doesn’t increase if the burning target is subjected to that trait again. Game features include spells, class features, feats, racial traits, monster abilities, and magic items. See the related rule in the “Combining Magical Effects” section of chapter 10 in the Player’s Handbook.
Does the humanoid take damage from both fireballs? or does the humanoid only take damage from the "most potent" fireball?