If you're using the same iTunes account, you can redownload your purchased content on a second device or a new Hero Academy account. Apple manages all of the receipts for purchases. If you tap "Buy" again on something you already purchased with that iTunes account, Apple will as for your iTunes/App Store login password and then after verifying the account it will tell you that you already own the content. You can unlock it from that dialogue box and will not be charged a second time (for single-purchase content like teams and avatar packs).
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