Xcode 10 Error: Multiple commands produce for 'app' and 'appex'

In case someone is still struggling with this after updating cocoapods and reinstalling pods:

1. Open Build Phases of the target which gives you problems
2. Open Copy Pods Resources phase.
3. Make sure the Output Files is empty. In my case I had a strange entry ${TARGET_BUILD_DIR}/${UNLOCALIZED_RESOURCES_FOLDER_PATH}. I removed it and the build was successful

Solved my problem by updating Cocoapods from 1.4.0 to 1.5.3