You don't need to pay twice as your license is normally valid forever without any time or download limit. All you need to do is to make sure, that you use the same market and the same account as for the initial purchase with your first device. Only then the market will remember you as a real owner of the application and will let you download it again for free to the new or reset devices.
Hint 1: Sometimes it might be necessary to clear the Play Store (or the other app store) cache/data. You can do it → Android OS Settings → Apps/Application Manager → All → Google Play Store → Clear Cache/Data and then - if necessary - remember to log in again with the right account.
Please remember, that the license is bound to the account and the app store and it is therefore not possible to transfer it either to the other email or to a different store.