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.

It's late but worth. I have done the same mistake. I added a project to firebase in the wrong account then I deleted and try to add in another firebase account but I couldn't do it because the error indicates that the project is already registered. So it takes time to round about 5 to 7 days to completely delete.

Here is the step I followed to generate another sha1.

1. Go to .android in my pc located. C:\Users\shahz.android.
2. delete debug.keystore.
3. rebuild android project.

4. click on signing report (right side bar click gradle, app/task/android/siginingReport)

5. you will get a new debug key with new sha1.

6. register your app.

As someone could wanna use the same application through two or more firebase projects for many reasons and so get this error I here do address this particular scenario. The esiest way to run the same application upon two or more different firebase projects (let’s say production and staging) is to add to your Module level build.gradle file a build variant (let's say staging) like this one:

apply plugin: 'com.android.application'
apply plugin: 'com.google.firebase.firebase-perf'

android {
    compileSdkVersion 27
    buildToolsVersion "27.0.3"
    defaultConfig {
        applicationId "com.mydomain.myapp"
        minSdkVersion 19
        targetSdkVersion 27
        versionCode 18
        versionName "2.8"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        multiDexEnabled true
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
        staging {
            initWith debug
            applicationIdSuffix ".staging"
            versionNameSuffix = "-staging"
        }
    }
}

dependencies {
    [...]
}

// Firebase
apply plugin: 'com.google.gms.google-services'

In the build variant staging what's most important is the line:

applicationIdSuffix ".staging"

This will inject at build time a ".staging" suffix to your application ID so that you will automatically have

applicationId "com.mydomain.myapp.staging"

You than need to add this application ("com.mydomain.myapp.staging") to your firebase staging project and so you will be able to add the same "com.mydomain.myapp" SHA1 to this application, because it has a different application ID.