Android build fails with Gradle dependency conflicts (Duplicate class)

First check: the class name in the error

The package name after Duplicate class is the clash itself. Repeated com.google.android.gms entries mean two modules demand different versions of the same library.

Pin the toolchain first

Use the JDK, SDK, and Gradle installed by Unity Hub. Unchecking them in Preferences > External Tools to use system versions doubles the number of causes.

Force one version

The most recent package (ads or analytics SDK) is the likely culprit. Remove packages one at a time to narrow it down, then enable Custom Main Gradle Template under Player Settings > Publishing Settings and enforce the version:

// mainTemplate.gradle
configurations.all {
    resolutionStrategy {
        force 'com.google.android.gms:play-services-base:18.2.0'
    }
}

Expected result and next step

A passing build means done. When two packages require the same library, unify toward the higher version. If it still fails, continue with the AAB keystore automation guide to isolate the Gradle stage.

> What went wrong:
Duplicate class com.google.android.gms.internal.base.zab
  play-services-base-18.0.1.jar and 18.2.0.jar