You should not need to purchase the game again, nor should you receive a prompt to purchase the game.
Are you playing on Android?
-- Did you previously have the demo installed?
-- Did Google Play update the game?
-- If you are playing on mobile, please try first uninstalling the game and any demos (you can make a backup of the saves if you do not want to lose them: instructions). When you reinstall the game, you should be prompted to *install* the game, not to *update the game.