Occasionally, even after purchasing an app on Google Play, and having confirmed you're signed in to the Play Store with the same account that was used for the purchase, you may be prompted to pay again for an app.
This problem can often be corrected by clearing the cached data for Google Play and restarting the device, and finally retrying to install the app from Google Play.
The steps to clear cached data are shown below (for Android 7).
1) Tap on Apps in Android Settings:
2) Scroll down to find "Google Play" and tap to select:
3) Tap on Storage to view options:
4) Choose "Clear Cache":
5) Finally, restart your device prior to attempting to install the app again.