The signing fingerprint you specified is already used by another Android OAuth2 client

This error occurs when there is an existing SIGNING CERTIFICATE FINGERPRINT (SHA1) on Google Developers linked to a different account.

Solution is to delete the project that is using that SHA1 in console.developers.google.com for the old/incorrect project.

You cannot change the SHA1 fingerprint once it is set. Remember that deleting the project takes 7 days to completely remove it.

Another option is to delete the debug.keystore and generate a new one with:

keytool -genkey -v -keystore debug.keystore -alias androiddebugkey
-storepass android -keypass android -keyalg RSA -validity 14000

Remember you have to uninstall the app otherwise you get the [INSTALL_FAILED_UPDATE_INCOMPATIBLE] error.

The signing fingerprint you specified is already used by another Android OAuth2 client

I have to check every project and tried to find another Client ID configured with the SHA1 fingerprint and package name that i tried to configure, unsuccesfully.

At the end the solution was delete the client ID that i tried to edit and add again with the SHA1 fingerprint and package name, it worked for me:

https://console.developers.google.com/apis/credentials

If you are using each firebase project for each env as I am, the below approach might be helpful.

In your build gradle, create new application id for each env:

productFlavors {
        dev {
            applicationId "se.abc.dev"
        }
        stag {
            applicationId "se.abc.stag"
        }
        prod
    }

On each firebase project, add a new project with corresponding application id with the same finger print. Remember to download the new google service json file since the application id was changed. The package name remains the same so it would not be a problem when uploading into google play. But for sure, I leave the prod flavor empty, so the package name and application id will be the same for prod release to avoid trouble.