Android build errorAndroid: “Duplicate class ... found in modules”

Where the second copy comes from, and the fix

1. 01

   Mixed Kotlin standard library versions

   “Note that mixing different versions of stdlib artifacts could lead to class duplication or to missing classes.” Kotlin, What’s new in Kotlin 1.8.0

   Since Kotlin 1.8, the contents of kotlin-stdlib-jdk7 and kotlin-stdlib-jdk8 live in kotlin-stdlib. A project or plugin that still pulls an older jdk8 artifact next to a new kotlin-stdlib gets the same classes twice. Update the Kotlin Gradle plugin (in Flutter, in android/settings.gradle, or ext.kotlin_version in android/build.gradle on older projects), update the libraries that pin old versions, and replace explicit jdk7 or jdk8 dependencies with kotlin-stdlib. The Kotlin BOM aligns the rest.

   // app/build.gradle.kts
   dependencies {
     implementation(platform("org.jetbrains.kotlin:kotlin-bom:1.9.24"))
   }

2. 02

   The same library is included twice

   “Your app has a local binary dependency and a remote binary dependency on the same library. To resolve this issue, remove one of the binary dependencies.” Android Developers, Fix dependency resolution errors

   A .jar or .aar copied into libs/ and the same library from Maven, or a library that already bundles another one you also declare, both put the classes on the classpath. Remove the direct dependency or the local file.

3. 03

   Find which dependency brings the copy

   “Type the name of the class that appears in the build error. Inspect the results for the dependencies that include the class.” Android Developers, Fix dependency resolution errors

   Print the dependency tree and search it for the module names from the error. In Android Studio, Navigate > Class with Include non-project items checked shows every library that contains the class.

   # from the android folder in Flutter and React Native projects
   cd android && ./gradlew app:dependencies --configuration releaseRuntimeClasspath | grep -n "stdlib"

4. 04

   Old support libraries next to AndroidX

   Duplicate class android.support.v4... next to androidx... means an old library still uses the Android Support Library. Set android.useAndroidX=true in gradle.properties, update the library to an AndroidX version, and enable android.enableJetifier=true only as a stopgap, because it slows every build.

   # android/gradle.properties
   android.useAndroidX=true
   android.enableJetifier=true  # temporary, until the old library is updated

5. 05

   Exclude the module from the dependency that brings it

   When a library you need brings an extra copy you cannot remove by upgrading, exclude that module from it. A common case is Guava’s listenablefuture clashing with a full Guava.

   // Kotlin DSL (build.gradle.kts)
   dependencies {
     implementation("some-library") {
       exclude(group = "com.google.guava", module = "listenablefuture")
     }
   }
   
   // Groovy (build.gradle, Flutter and React Native)
   implementation('some-library') {
     exclude group: 'com.google.guava', module: 'listenablefuture'
   }

Checked against the vendor’s docs on November 7, 2026: Android Developers: Fix dependency resolution errors (duplicate classes); Kotlin: What’s new in Kotlin 1.8.0 (jdk7 and jdk8 merged into kotlin-stdlib).

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Related: Flutter Kotlin Gradle plugin version · Android “Manifest merger failed” · Gradle “Could not resolve all files”