I bought a managed IAP (e.g. the Coin Doubler) from Google Play, but how do I get that again on my new device?

If you are logged into the Play Store with the same account you used when you originally purchased the item, you should be able to click the item (e.g. Coin Doubler) again, and it will give it to you without re-charging you.

Please note that only managed items like the Coin Doubler or Go Premium can be restored.  Consumables like coin or gem packs cannot be restored.

You can also try the following:

1. Check that you are logged into the same Google Play account which you used to make your original purchases.
2. Ensure you have the latest update for the game
3. Ensure you have the latest operating system for your device
4. from Google support page:

Clear the cache & data of the Play Store

This gives the app a fresh start and can help fix issues.

1. On your Android phone or tablet, open the Settings app .

2. Tap Apps & notifications  See all apps.

3. Scroll down and tap Google Play Store .

4. Tap Storage  Clear Cache.

5. Tap Clear data.

6. Re-open the Play Store, and try your download again.