sending message to one branch / topic

Hi. Guys, please tell me how to do the following:

I have a site with registered users. Every month I send them a service message from the admin. How to make sure that a new topic is not created every time. and the message was sent to one branch / topic

now I sends like this and after every message creates new topic:

        // Prepare message data

        $message = array(

             'message_title' => 'Your monthly fees ' . fep_get_userdata( $user_id, 'display_name', 'id' ), //change with message title

             'message_content' => 'body message....', //change with message content

             'message_to_id' => $user->ID
        );

        $override = array(

             'mgs_author' => 6, //change with message sender id

        );

        // Send message
       fep_send_message( $message, $override );