Purchases made on Google Play are linked to your Google account. If you buy a new smartphone and login with the same Google account, you can install the app again, without needing to buy it again.
The same applies if you want to install the app on your tablet and smartphone at once. This is possible, if you use the same Google account.
You can find more information about this in the Google Play Help.
Please note: we follow the official Google Play guidelines. If you are unsure, we recommend to contact the Google Play support.