Try to delete your build folder in app/build. And then try a clean build

Might be you have not enabled multidex,

Please go to app/build.gradle, if it is not enabled then enable it and restart the app again.

here is how you can do that:

Goto ->

/android/app/build.gradle and add

multidrxEnabled true

In defaultConfig

It should look like this:

defaultConfig {
            // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
            applicationId "com.example.chat_app_flutter_firebase"
            minSdkVersion 16
            targetSdkVersion 28
            versionCode flutterVersionCode.toInteger()
            versionName flutterVersionName
            testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

            //Enable multidex by adding this line 

            multiDexEnabled true
        }

I was trying to add Firebase Crashlytics to my app and I followed this tutorial. When I ran the app, I am getting the following duplication runtime exception.

Execution failed for task ':app:mergeDebugResources'.
> [string/com.crashlytics.android.build_id] /home/chinkysight/Desktop/Pecha/build/app/generated/fabric/res/debug/values/com_crashlytics_build_id.xml    [string/com.crashlytics.android.build_id] /home/chinkysight/Desktop/Pecha/build/app/generated/crashlytics/res/debug/values/com_crashlytics_build_id.xml: Error: Duplicate resources