The Full Version of iStudiez Pro for iOS allows you to sync on your iOS devices.
So there's no need to buy the Full Version separately for iPhone and iPad.
If you have already bought the Full Version for iOS on one device (e.g. on iPhone), but asked to buy it again on another (iPad), please reinstall the app on a device where you face the issue.
Make sure you are signed into the App Store under the same Apple ID that was used to buy the Full Version for iOS.
Don't forget to create iStudiez Pro data backup before the reinstall.
If you reinstalled the app, but still asked to buy it again, tap the "Buy Full Version" button and you should not be charged.
Otherwise, please contact Apple Support Team for the assistance.