react native inline vs end of file - export default - while using firebase

Question

i am using firebase for my app's authentication. when i run my code, emulator runs it without a problem but when i change something in my code and check my emulator, depending on my code an error occurs. if i use:

      export default class App extends Component{....

refreshing causes a problem FirebaseError: Firebase: Firebase App named '[DEFAULT]' already exists (app/duplicate-app).

but if i use export in the end like this:

       export default App;

than it's fine. I saw many comments about these 2 approaches being almost same but i couldn't understand what makes the difference here, btw i am connecting to firebase this way:

componentDidMount() {
   firebase.initializeApp({configs});
}