// helper function to prevent duplicate output filenames
  auto hasAppender = [&existingAppenders](std::string const& filename) {
    if (existingAppenders.find(filename) != existingAppenders.end()) {
      return true;
    }
    // treat stderr and stdout as one output filename
    if (filename == "-" &&
        existingAppenders.find("+") != existingAppenders.end()) {
      return true;
    }
    if (filename == "+" &&
        existingAppenders.find("-") != existingAppenders.end()) {
      return true;
    }
    return false;
  };

  if (hasAppender(filename)) {
    // already have an appender for the same output
    return nullptr;
  }