Error:Execution failed for task ':app:transformClassesWithJarMergingForDebug'

In terminal execute in root project folder:

./gradlew clean

It helped me.

AndroidStudio Menu:

Build/Clean Project

Update old dependencies

All the above not working for me.. Because I am using Facebook Ad dependency..

Incase If anybody using this dependency compile 'com.facebook.android:audience-network-sdk:4.16.0'

Try this code instead of above

compile ('com.facebook.android:audience-network-sdk:4.16.0'){
exclude group: 'com.google.android.gms'
}