Example of a Podfile resulting in errors due to classes implemented in multiple targets (https://github.com/CocoaPods/CocoaPods/issues/6918)

Podfile

# Uncomment the next line to define a global platform for your project
platform :ios, '9.0'

target 'EarthCore' do
    # Comment the next line if you're not using Swift and don't want to use dynamic frameworks
    use_frameworks!

    # Pods for EarthCore
    pod 'Firebase', '~> 4.0', :subspecs => ['Core', 'Database', 'Messaging', 'Auth', 'Storage', 'Performance']

    target 'EarthCoreTests' do
        inherit! :search_paths
        # Pods for testing
    end

end

target 'Earthlings' do
    # Comment the next line if you're not using Swift and don't want to use dynamic frameworks
    use_frameworks!

    # Pods for Earthlings

    target 'EarthlingsTests' do
        inherit! :search_paths
        # Pods for testing
    end
    
end