We apologize for the inconvenience.
In rare cases, Apple's payment processing does not complete correctly. As long as you use the same Apple Account, you will never be charged twice. Restart the app, go to the Premium screen, and tap the purchase button again to run the payment process once more.
If the previous payment had failed, the purchase will simply go through correctly this time. If the previous payment had actually succeeded, a message will appear saying that you have already purchased it and it will be restored, and Premium will be applied.
Thank you for checking.