Net::NNTP.start(news-europe.giganews.com,119,USER,PASSWORD) do |nntp|
...
end

Throws error "502 already authenticated" when logging in for the first time.

My Workaround:

class Net::NNTP
  # added the line
  # return if stat == "502 already authenticated\n"
  # for compatibilty with giganews.
  def auth_original(user, secret)
    stat = critical {
      check_response(get_response("AUTHINFO USER %s", user), true)
      check_response(get_response("AUTHINFO PASS %s", secret), true)
    }
    return if stat == "502 already authenticated\n"
    raise NNTPAuthenticationError, stat unless /\A2../ === stat
  end
end

Now it works fine, but I think this solution is not ideal