Hey there! I see you're new to Android and Flutter and ran into this duplicate class issue—don't worry, this is a super common problem when mixing old and new Google Play Services/Firebase dependencies, and we can fix it step by step.

What's Causing the Issue? #

The error boils down to conflicting versions of Google Play Services/Firebase modules:

• Your project directly uses the monolithic play-services:12.0.1 package (a bloated bundle that includes dozens of sub-modules like places, measurement, etc.)
• Your Flutter dependencies (like firebase_core, google_sign_in, google_maps_flutter) automatically pull in newer versions of these same sub-modules.

These overlapping, mismatched versions create duplicate class definitions that Gradle can't resolve.

Step-by-Step Fix #

1. Replace the Monolithic Play Services Dependency #

Instead of using the entire play-services package (which is outdated and adds unnecessary bloat), use only the specific sub-modules your app actually needs. For example, if you're using location features, replace the full package with the location sub-module.

Update the dependencies section in your app/build.gradle:

dependencies {
    implementation 'com.android.support:multidex:1.0.3'
    // Replace the full play-services package with targeted sub-modules
    implementation 'com.google.android.gms:play-services-location:17.1.0' // Match version with Flutter's dependencies
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
}

Note: If you need other sub-modules (like maps or auth), add them individually. Check the official Google Play Services docs for the latest compatible versions.

2. Update Google Services Plugin Version #

Your root build.gradle uses an older version of the Google Services plugin that might not play nice with newer Firebase dependencies. Update it to a more recent compatible version:

buildscript {
    ext.kotlin_version = '1.3.50'
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:4.0.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        // Updated to a newer compatible version
        classpath 'com.google.gms:google-services:4.3.15'
    }
}

3. Clean and Rebuild the Project #

After making these changes, clear cached dependencies and rebuild to ensure everything syncs properly:

flutter clean
flutter pub get
flutter run

Verify Dependencies (Optional) #

If you want to double-check for remaining conflicts, run this command to see all resolved dependencies:

flutter pub deps

This will show you exactly which versions of each module are being used, so you can confirm no mismatches remain.

Original Files for Reference #

app/build.gradle (Original)

def localProperties = new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {
    localPropertiesFile.withReader('UTF-8') { reader -> localProperties.load(reader) }
}
def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}
def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
    flutterVersionCode = '1'
}
def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
    flutterVersionName = '1.0'
}
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
android {
    compileSdkVersion 30
    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
    }
    lintOptions {
        disable 'InvalidPackage'
    }
    defaultConfig {
        applicationId "com.example.ipill"
        minSdkVersion 25
        targetSdkVersion 30
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
        multiDexEnabled true
    }
    buildTypes {
        release {
            signingConfig signingConfigs.debug
        }
    }
}
flutter {
    source '../..'
}
dependencies {
    implementation 'com.android.support:multidex:1.0.3'
    implementation 'com.google.android.gms:play-services:12.0.1'
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
}
apply plugin: 'com.google.gms.google-services'

Root build.gradle (Original)

buildscript {
    ext.kotlin_version = '1.3.50'
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:4.0.1'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath 'com.google.gms:google-services:4.3.3'
    }
}
allprojects {
    repositories {
        google()
        jcenter()
    }
}
rootProject.buildDir = '../build'
subprojects {
    project.buildDir = "${rootProject.buildDir}/${project.name}"
}
subprojects {
    project.evaluationDependsOn(':app')
}
task clean(type: Delete) {
    delete rootProject.buildDir
}

Error Log

* What went wrong:
Execution failed for task ':app:checkDebugDuplicateClasses'.
> 1 exception was raised by workers:
java.lang.RuntimeException: java.lang.RuntimeException: Duplicate class com.google.android.gms.location.places.zza found in modules jetified-play-services-places-12.0.1-runtime.jar (com.google.android.gms:play-services-places:12.0.1) and jetified-play-services-places-placereport-17.0.0-runtime.jar (com.google.android.gms:play-services-places-placereport:17.0.0)
Duplicate class com.google.android.gms.measurement.AppMeasurement found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-impl-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-impl:17.4.1)
Duplicate class com.google.android.gms.measurement.AppMeasurement$ConditionalUserProperty found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-impl-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-impl:17.4.1)
Duplicate class com.google.android.gms.measurement.AppMeasurement$Event found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-impl-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-impl:17.4.1)
Duplicate class com.google.android.gms.measurement.AppMeasurement$EventInterceptor found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-impl-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-impl:17.4.1)
Duplicate class com.google.android.gms.measurement.AppMeasurement$OnEventListener found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-impl-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-impl:17.4.1)
Duplicate class com.google.android.gms.measurement.AppMeasurement$Param found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-impl-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-impl:17.4.1)
Duplicate class com.google.android.gms.measurement.AppMeasurement$UserProperty found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-impl-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-impl:17.4.1)
Duplicate class com.google.firebase.analytics.FirebaseAnalytics found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-api-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-api:17.4.1)
Duplicate class com.google.firebase.analytics.FirebaseAnalytics$Event found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-api-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-api:17.4.1)
Duplicate class com.google.firebase.analytics.FirebaseAnalytics$Param found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-api-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-api:17.4.1)
Duplicate class com.google.firebase.analytics.FirebaseAnalytics$UserProperty found in modules jetified-firebase-analytics-impl-12.0.1-runtime.jar (com.google.firebase:firebase-analytics-impl:12.0.1) and jetified-play-services-measurement-api-17.4.1-runtime.jar (com.google.android.gms:play-services-measurement-api:17.4.1)

Flutter pubspec.yaml Dependencies

flutter:
  sdk: flutter
cupertino_icons: ^0.1.3
firebase_core: ^0.5.0
firebase_auth: ^0.18.0+1
cloud_firestore: ^0.14.0+2
google_sign_in: ^4.5.3
google_maps_flutter: ^0.5.27+3
http: ^0.12.0+1
json_serializable: ^3.3.0
geolocator: ^6.0.0
google_place: ^0.3.3
dev_dependencies:
  pedantic: ^1.9.0
  build_runner: ^1.10.0
  flutter_test: sdk: flutter

内容的提问来源于stack exchange，提问作者Alex97