Boost.Log: Why it is showing duplicate the message?

I'm learning the Boost.Log library. I want to send messages to a file and std::clog. I have the following class:

class logger
{
    public:
        explicit logger(
            const std::string& tag,
            const std::string& file,
            bool console
        )
        {
            boost::log::register_simple_formatter_factory< boost::log::trivial::severity_level, char >("Severity");

            std::string the_format = "[%TimeStamp%] (%LineID%) [%Severity%]";
            if(!tag.empty()) {
                m_log_.add_attribute(
                    "Tag",
                    boost::log::attributes::constant< std::string >( tag )
                );
                the_format += " [%Tag%]";
            }

            the_format += ": %Message%";

            if(console) {
                boost::log::add_console_log(
                    std::clog,
                    boost::log::keywords::auto_flush = true,
                    boost::log::keywords::format = the_format
                );
            }

            if(!file.empty()) {
                boost::log::add_file_log(
                    boost::log::keywords::file_name = file,
                    boost::log::keywords::auto_flush = true,
                    boost::log::keywords::open_mode = (std::ios::out | std::ios::app),
                    boost::log::keywords::format = the_format
                );
            }

        }

        ~logger(void)
        { }

        void log(
            const std::string& msg
        )
        {
            BOOST_LOG_SEV ( m_log_, boost::log::trivial::info ) << msg;
        }

    private:
        boost::log::sources::severity_logger< boost::log::trivial::severity_level > m_log_;

}; // logger

I have the following main() function:

int main(int argc, char** argv)
{
    logger lg( "alfa", "", true );
    lg.log( "Hello world!!!");
    return 0;
}

I do not understand why the duplicate message is displayed: "Hello world!!!":

[2016-Aug-23 16:34:22.959242] (1) [info] [alfa]: Hello world!!!
[2016-Aug-23 16:34:22.959242] (1) [info] [alfa]: Hello world!!!