Duplicate class com.google.android.gms.internal.firebase_messaging.zza found in modules

In build.gradle:

implementation "com.google.firebase:firebase-iid:21.1.0"

None of the answers above is correct. And none of them solves the problem. The problem is in transitive dependencies. You need to check which libs are causing the issue and do something like:

implementation 'com.google.firebase:firebase-messaging:22.0.0'
implementation ('com.google.firebase:firebase-iid:21.1.0') {
    transitive = true
}

I resolved this problem by using the Firebase Android BoM

In my case the problem depends on the coexistence of messaging and functions.

So from this:

implementation 'com.google.firebase:firebase-messaging:22.0.0'
implementation 'com.google.firebase:firebase-functions-ktx:20.0.0'

To this:

implementation platform('com.google.firebase:firebase-bom:28.2.1')
implementation 'com.google.firebase:firebase-functions-ktx'
implementation 'com.google.firebase:firebase-messaging'

I encountered the same problem.

I fixed it by redefining my dependencies implementations like this:

In build.gradle:

implementation 'com.google.firebase:firebase-analytics:19.0.0'
implementation 'com.google.firebase:firebase-auth:17.0.0'
implementation 'com.google.firebase:firebase-messaging:22.0.0'

Please note my configuration in buildscript: com.android.tools.build:gradle:4.2.1

Then, clean and rebuild project.

Can you share more information about your setup?

Using react-native-push-notification ^3.1.9, I've got the same error.

To fix it :

in project/build.gradle:

buildscript {
    ext {
        googlePlayServicesVersion = "17.0.0"
        firebaseMessagingVersion = "20.1.0"
        firebaseVersion = "20.1.0"

        // other settings
    }
 // other settings
}

in project/app/build.gradle:

dependencies {
    implementation "com.google.firebase:firebase-messaging:20.1.0"
    // other implementations
}

The reason : react-native-push-notification package specify dependency to firebase-messaging:+ : which ask to get the last version. So, gradle upload the latest com.google.firebase:firebase-messaging:22.x.x which declare already existing classes in firebase-iid.

If you don't want to use firebase-bom because you don't trust firebase to keep all the proper versions in check then you can remove the duplicate.

1. Run ./gradlew app:dependencies (or replace app with your module)
2. Search for iid:19.0.0 (or whatever version is giving you issues)
3. Scroll up in the list to see where the library originates from.

You should see something like \--- com.google.firebase:firebase-iid:19.0.0

4. Keep scrolling until you find the dependency and module it originates in, marked with a + eg (+--- com.google.firebase:firebase-ml-vision)
5. Locate the dependency in your project and remove the duplicate iid

api("com.google.firebase:firebase-ml-vision:$version") {
    exclude group: 'com.google.firebase', module: 'firebase-iid'
}