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With the release of the new expansion several new cards appeared in my collection out of nowhere, how is that possible and what caused it? I'm not talking just about the rebalanced alchemy cards, for example I got a copy of Atsushi from kamigawa that I'm pretty sure I didn't have before. I mean, I ain't complaining but it's weird. My wildcards and other resources looked the same I think so it's not someone who accessed my account and spent some of my stuff.

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over 2 years ago - /u/WotC_Jay - Direct link

Before rotation each year we refresh the 10 dual-color starter decks and ensure that all players have the cards needed to play them (so everyone always has decks they can play in the rotating formats). You're probably seeing the new cards added for that, and you can find those new decks in your Starter Decks folder.

See the "New Player Decks" section here for more info on that: https://magic.wizards.com/en/articles/archive/magic-digital/mtg-arena-renewal-rotation