In games that offer online fights, especially the more popular ones, you will from time to time run into a cheater. Fire Emblem Heroes is no exception, as you may struggle against teams line up several of the same character even though they are supposed to be unique ...
But it's now over because Nintendo has decided to take action via an update that will send a "To You" notification as a warning to the hackers involved. They will experience temporary limitations on several features of the game. More details expected soon.
As a reminder, Fire Emblem Heroes is available as a free download on iOS and Android, but features in-app purchases.