Dealing with Email address already in use - Firebase Authentication

There are 3 ways in which you can handle this problem.

The first one is to verify if the email address exists and than display a message. This is exactly what you said. The message is up to you.

The second approach is to enable users to have multiple accounts per email address. With other words, if a user signs up with gmail and then signs up with Facebook and he has the same email address, than he ends up having 2 different accounts. A single email address, 2 different accounts This is not a good practice but according to your needs, you can even use it.

The third approach is to have only one account per email address. This means that you are preventing the users from creating multiple accounts using the same email address with different authentication providers. This a common practice and also the default rule in the Firebase console. This means, that you'll want to implement later another kind of authentication with another provider, it will follow the same rule. In this case, will have a single email address with a single account.

To enable or disable this option, go to your Firebase console, choose Authentication, select the SIGN-IN METHOD tab and at the bottom of your page you'll find the Advanced section.