I purchased the app before, but I keep being prompted to purchase the app again

When prompted to purchase the app, you should scroll to the bottom and tap the “Restore Purchase” option at the bottom.
Just make sure that you are using the same Apple ID as the one you used to purchase the app with.
Also, notice that if you purchased the “Pro” version, you should make sure you download the “Pro” version from the App Store (The ‘Pro’ and the ‘non-Pro’ versions are the same app with the same features).

Feedback and Knowledge Base