com.android.builder.dexing.DexArchiveMergerException: Unable to merge dex - Android Studio 3.0 stable

I know it's too late to update.I had same issue on my project.

Possible Reasons

1. If you have added module in your project and that module has support libraries or any google play services libs which has different version then your app.
2. If you are using any open source library in your project and that library internally using any of libraries that your are also using in your project.

Solutions

• If it is case 1 in your project then update your library versions and make it same in your project and module.

• Check your dependencies tree using below command and see if any mismatch in dependencies.

  ./gradlew :app:dependencies
  

• You can exclude particular module from any dependencies like below.

  implementation('com.google.android.ads.consent:consent-library:1.0.4') {
    transitive = true
    exclude group: "com.android.support"
  } 
  

• In above example, It will exclude the com.android.support group from consent-library dependencies.

• You can also remove particular module as well.

   compile ('junit:junit:4.12'){
    exclude group: 'org.hamcrest', module:'hamcrest-core'
    }
  

• In above example it will exclude hamcrest-core from org.hamcrest.

1. Go to <project>/app/ and open build.gradle file
2. Add the following line to the defaultConfig and dependencies sections

android {
  ...

  defaultConfig {
    ...
    multiDexEnabled true // ADD THIS LINE
  }
}

...

dependencies {
  ...
  implementation 'com.android.support:multidex:1.0.3'  // ADD THIS LINE
}

I got the same problem, adding sourceCompatibility and targetCompatibility to my build.gradle helped me:

android {
    compileOptions {
        sourceCompatibility 1.8
        targetCompatibility 1.8
    }
}