If you purchase an app on Google Play and then try to install it on a different device, Google Play may ask you to purchase the app again. If this happens, please give Google Play 24-48 hours to sync your purchase across all devices.
Per Google Play policy, you only need to purchase once and you can then install on as many devices as you want that use the same Google Play account. If Google Play isn’t allowing that, please give them a day or so to get their act together. Please do not purchase again.