I paid but the app is asking me to pay again. How do I log in?

The problem might arise because instead of clicking on the 'I already have an account' button, on the first screen of the app, which is for the users who already have an account, you might be pushing the 'Get started' button, which is for the new users only.

So when you open the app, it's the 'I already have an account' button that you should push, as shown below:

If you're doing that, you'll be taken to the next screen, where you'll need to insert your email address and password. After that, you'll be in the app that will show you the Dashboard screen.