Gradle fails with Duplicate Classes error when using Firebase Cloud Messaging and sceneform:core in mainTemplate.gradle

[Vangaorth]

[REQUIRED] Please fill in the following fields:

• Unity editor version: 2019.4.21
• Firebase Unity SDK version: 7.1.0
• Source you installed the SDK: Unity Package Manager
• Problematic Firebase Component: Cloud Messaging
• Other Firebase Components in use: App (Core), Crashlytics, Analytics
• Additional SDKs you are using: Third party remote assistance library (as .AAR + dependencies in mainTemplate.gradle)
• Platform you are using the Unity editor on: Windows
• Platform you are targeting: iOS and Android
• Scripting Runtime: IL2CPP

[REQUIRED] Please describe the issue here:

When using the Firebase Cloud Messaging (through the Unity Package Manager), having a custom mainTemplate.gradle where a third party dependencies to com.google.ar.sceneform:core is specified (implementation "com.google.ar.sceneform:core:1.17.1"), the build process fails during "Building GRadle project" phase with the following errors:

java.lang.RuntimeException: java.lang.RuntimeException: Duplicate class com.google.flatbuffers.Constants found in modules classes.jar (com.google.ar.sceneform:sceneform-base:1.17.1) and classes.jar (com.google.firebase:firebase-messaging-unity:7.1.0)
Duplicate class com.google.flatbuffers.FlatBufferBuilder found in modules classes.jar (com.google.ar.sceneform:sceneform-base:1.17.1) and classes.jar (com.google.firebase:firebase-messaging-unity:7.1.0)
Duplicate class com.google.flatbuffers.FlatBufferBuilder$ByteBufferFactory found in modules classes.jar (com.google.ar.sceneform:sceneform-base:1.17.1) and classes.jar (com.google.firebase:firebase-messaging-unity:7.1.0)
Duplicate class com.google.flatbuffers.FlatBufferBuilder$HeapByteBufferFactory found in modules classes.jar (com.google.ar.sceneform:sceneform-base:1.17.1) and classes.jar (com.google.firebase:firebase-messaging-unity:7.1.0)
Duplicate class com.google.flatbuffers.Struct found in modules classes.jar (com.google.ar.sceneform:sceneform-base:1.17.1) and classes.jar (com.google.firebase:firebase-messaging-unity:7.1.0)
Duplicate class com.google.flatbuffers.Table found in modules classes.jar (com.google.ar.sceneform:sceneform-base:1.17.1) and classes.jar (com.google.firebase:firebase-messaging-unity:7.1.0)
Duplicate class com.google.flatbuffers.Utf8 found in modules classes.jar (com.google.ar.sceneform:sceneform-base:1.17.1) and classes.jar (com.google.firebase:firebase-messaging-unity:7.1.0)
Duplicate class com.google.flatbuffers.Utf8Safe found in modules classes.jar (com.google.ar.sceneform:sceneform-base:1.17.1) and classes.jar (com.google.firebase:firebase-messaging-unity:7.1.0)

This does not happen in a native Android Studio project (no Unity involved), where the app.gradle dependencies are as follows:

implementation "com.google.ar.sceneform:core:1.17.1"
implementation platform('com.google.firebase:firebase-bom:26.6.0')
implementation 'com.google.firebase:firebase-messaging'

Steps to reproduce:

• Clone https://github.com/Vangaorth/UnityFCMDuplicateClasses, switch to Android Platform, build an APK

Or

• Create a new Unity Android project, add the Firebase Cloud Messaging (7.1.0) using the Unity Package Manager (when asked from the External Dependency Manager, specify "Enable")
• In Player Settings -> Publishing Settings, specifiy that you want to use a custom mainTemplate.gradle
• Add "implementation "com.google.ar.sceneform:core:1.17.1" to the dependencies of mainTemplate.gradle
• Build an APK

[vimanyu]

Thank you for providing details and clear steps for reproduction. I can see this error in our messaging quickstart example in Unity 2020 and Firebase Unity plugins 7.1.0. Filing this as a bug.