Did you get duplicate stickers? Perfect! For each duplicate stickers you will get duplicate stars depending on how rare your duplicated sticker was. For example, if you get a repeated 2 star sticker, you will get 2 duplicated stars in exchange of it. After you got a nice amount of stars, you can spend your duplicate stars on bundles in the Star Shop inside the Sticker album screen.
Be careful though! These duplicate stars will be reset once the season of the Sticker album is over, so be sure to use them before that!