It says I haven't purchased this app before, but I have

If you're trying to reinstall the app you downloaded in the past and the App Store shows you haven't downloaded or purchased that app before, please check to confirm you're downloading the correct version of the app. Each of our apps is available in two versions: (1) a free version with an in-app upgrade, and (2) a pay-up-front "Pro" version (with a yellow banner on the icon). The upgraded free version and the pay-up-front Pro versions are equivalent, but they are not interchangeable. So make sure you're downloading the same version you had before. (If you’re not sure, you can review your iTunes purchase history.)

If you’re reinstalling the free version, to restore the upgrade:
  1. Go to the Settings tab in the app.
  2. Tap Restore In-App Purchase.
If iTunes isn't recognizing your past purchase, even though you're trying to download or restore the correct version, check to make sure you're signed in to the same iTunes account you used to download or purchase the item originally.

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