What causes the 'Duplicate class' error in Singular setup for Flutter apps, and how to fix it?

Question

Implementing Singular in Flutter application and getting sdk setup error.

Execution failed for task ':app:checkDebugDuplicateClasses'.
> A failure occurred while executing com.android.build.gradle.internal.tasks.CheckDuplicatesRunnable
   > Duplicate class com.google.android.gms.actions.ReserveIntents found in modules jetified-play-services-6.5.87-runtime (com.google.android.gms:play-services:6.5.87) and jetified-play-services-basement-18.1.0-runtime (com.google.android.gms:play-services-basement:18.1.0)

https://support.singular.net/hc/en-us/articles/4408894547227-Singular-SDK-Integration-for-Flutter

Following this link to set up Singular and it showing me error for Android duplicate class

app/build.gradle:

    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'
    // START: FlutterFire Configuration
    apply plugin: 'com.google.gms.google-services'
    // END: FlutterFire Configuration
    apply plugin: 'kotlin-android'
    apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"
    
    android {
        compileSdkVersion flutter.compileSdkVersion
        ndkVersion flutter.ndkVersion
    
        compileOptions {
            sourceCompatibility JavaVersion.VERSION_1_8
            targetCompatibility JavaVersion.VERSION_1_8
        }
    
        kotlinOptions {
            jvmTarget = '1.8'
        }
    
        sourceSets {
            main.java.srcDirs += 'src/main/kotlin'
        }
    
        defaultConfig {
            // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
            applicationId "com.example.tasks_app"
            // You can update the following values to match your application needs.
            // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-gradle-

build-configuration.
        minSdkVersion 19
        targetSdkVersion flutter.targetSdkVersion
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
        multiDexEnabled = true
    }

    buildTypes {
        release {
            // TODO: Add your own signing config for the release build.
            // Signing with the debug keys for now, so `flutter run --release` works.
            signingConfig signingConfigs.debug
        }
    }
}

flutter {
    source '../..'
}


dependencies {
    implementation 'com.google.android.gms:play-services:12.0.1'
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:28.0.0'
    implementation 'com.google.android.gms:play-services-ads-identifier:17.0.0+'

    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
}

From the documentation I followed the steps but Firebase class is clashing I don't understand much as I am first time trying to implement this type of sdk for analytics purpose and I want to learn about Singular