You can only have one Mega Pokémon at a time.
In case you Mega Evolve a different one, your first Pokémon will revert back to normal and you’ll need to collect Mega Energy to evolve it again.
Using a Mega Pokémon in a raid gives other Pokémon an attack boost, with an additional attack boost given to Pokémon of the same type. These boosts do not stack.
Your Pokémon will only be Mega Evolved for a time period, which you can check on the information screen for that Pokémon.
Once it returns to normal, it will be significantly cheaper to Mega Evolve it again.
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