Class is implemented in both, One of the two will be used. Which one is undefined

I don't know the reason, but if you open your app's Pods-[AppName].debug.xcconfig file that cocoapods creates you'll find OTHER_LDFLAGS and you'll see it links to the same frameworks you link in your framework. So if you remove -framework [Duplicated framework] the warning disappears.

Seems to be a cocoapods error

I also found another a script someone wrote that fix the bug automatically. It's simply make the same I answered above. Add it to your Podfile:

post_install do |installer|
    sharedLibrary = installer.aggregate_targets.find { |aggregate_target| aggregate_target.name == 'Pods-[MY_FRAMEWORK_TARGET]' }
    installer.aggregate_targets.each do |aggregate_target|
        if aggregate_target.name == 'Pods-[MY_APP_TARGET]'
            aggregate_target.xcconfigs.each do |config_name, config_file|
                sharedLibraryPodTargets = sharedLibrary.pod_targets
                aggregate_target.pod_targets.select { |pod_target| sharedLibraryPodTargets.include?(pod_target) }.each do |pod_target|
                    pod_target.specs.each do |spec|
                        frameworkPaths = unless spec.attributes_hash['ios'].nil? then spec.attributes_hash['ios']['vendored_frameworks'] else spec.attributes_hash['vendored_frameworks'] end || Set.new
                        frameworkNames = Array(frameworkPaths).map(&:to_s).map do |filename|
                            extension = File.extname filename
                            File.basename filename, extension
                        end
                    end
                    frameworkNames.each do |name|
                        if name != '[DUPLICATED_FRAMEWORK_1]' && name != '[DUPLICATED_FRAMEWORK_2]'
                            raise("Script is trying to remove unwanted flags: #{name}. Check it out!")
                        end
                        puts "Removing #{name} from OTHER_LDFLAGS"
                        config_file.frameworks.delete(name)
                    end
                end
            end
            xcconfig_path = aggregate_target.xcconfig_path(config_name)
            config_file.save_as(xcconfig_path)
        end
    end
end

Updated: I write a blog article for my solution: https://medium.com/@GalvinLi/tinysolution-fix-cocoapods-duplicate-implement-warning-5a2e1a505ea8

And a demo project: https://github.com/bestwnh/TinySolution

---

I got the solution idea from internet but can't find one solution work, so I make a workaround by myself. Maybe the code is a bit long, but it work. Hope it can help someone.

The auto_process_target(,,) is the key function, just change it to fit your project and all should work fine. (Because I use one framework for multi app target, so I make the app target parameter be an array.)

post_install do |installer|

  # you should change the sample auto_process_target method call to fit your project

  # sample for the question
  auto_process_target(['MyFrameworkExampleApp'], 'MyFramework', installer)
  # sample for the multi app use on same framework
  auto_process_target(['exampleiOSApp', 'exampleMacApp'], 'exampleFramework', installer)

end

# the below code no need to modify

def auto_process_target(app_target_names, embedded_target_name, installer)
  words = find_words_at_embedded_target('Pods-' + embedded_target_name,
                                        installer)
  handle_app_targets(app_target_names.map{ |str| 'Pods-' + str },
                     words,
                     installer)
end

def find_line_with_start(str, start)
  str.each_line do |line|
      if line.start_with?(start)
        return line
      end
  end
  return nil
end

def remove_words(str, words)
  new_str = str
  words.each do |word| 
    new_str = new_str.sub(word, '')
  end
  return new_str
end

def find_words_at_embedded_target(target_name, installer)
  target = installer.pods_project.targets.find { |target| target.name == target_name }
  target.build_configurations.each do |config|
    xcconfig_path = config.base_configuration_reference.real_path
    xcconfig = File.read(xcconfig_path)
    old_line = find_line_with_start(xcconfig, "OTHER_LDFLAGS")

    if old_line == nil 
      next
    end
    words = old_line.split(' ').select{ |str| str.start_with?("-l") }.map{ |str| ' ' + str }
    return words
  end
end

def handle_app_targets(names, words, installer)
  installer.pods_project.targets.each do |target|
    if names.index(target.name) == nil
      next
    end
    puts "Updating #{target.name} OTHER_LDFLAGS"
    target.build_configurations.each do |config|
      xcconfig_path = config.base_configuration_reference.real_path
      xcconfig = File.read(xcconfig_path)
      old_line = find_line_with_start(xcconfig, "OTHER_LDFLAGS")

      if old_line == nil 
        next
      end
      new_line = remove_words(old_line, words)

      new_xcconfig = xcconfig.sub(old_line, new_line)
      File.open(xcconfig_path, "w") { |file| file << new_xcconfig }
    end
  end
end

If all work. You will see Update xxxx OTHER_LDFLAGS when you pod install or pod update. Then the warning gone.