Commenting on a post from the admin panel?

Question

This creates a custom meta box and the contents of the textarea field it contains are being saved as a comment to the post. Please note:

Built with code sample adapted from the Codex.
Relevant functions: add_meta_box and wp_insert_comment.
I removed comment_author_IP and comment_agent from the insert comment arguments. Not sure of the consequences.
Bug: duplicated comments. Post revisions they were, thanks to @Hameedullah Khan…
Tested as a mu-plugin and inside the theme’s functions.php

add_action( 'add_meta_boxes', 'add_custom_box_wpse_81739' );
add_action( 'save_post', 'save_postdata_wpse_81739', 10, 2 );

/* Adds a box to the main column on the Post and Page edit screens */
function add_custom_box_wpse_81739() 
{
    add_meta_box( 
        'sectionid_wpse_81739',
        __( 'Publish a Comment' ),
        'inner_custom_box_wpse_81739',
        'post' 
    );
}

/* Prints the box content */
function inner_custom_box_wpse_81739( $post ) 
{
    // Use nonce for verification
    wp_nonce_field( plugin_basename( __FILE__ ), 'noncename_wpse_81739' );

    // The actual fields for data entry
    // Use get_post_meta to retrieve an existing value from the database and use the value for the form
    //$value = get_post_meta( $post->ID, $key = '_my_meta_value_key', $single = true );
    echo '<label for="new_field_wpse_81739">';
    _e( "Enter your comment:" );
    echo '<br />';
    echo '<textarea id="new_field_wpse_81739" name="new_field_wpse_81739" cols="80" rows="5"></textarea></label>';
}

/* When the post is saved, saves our custom data */
function save_postdata_wpse_81739( $post_id, $post_object ) 
{
    // verify if this is an auto save routine. 
    // If it is our form has not been submitted, so we dont want to do anything
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE )  
        return;

    // don't run the echo if the function is called for saving revision.
    if ( 'revision' == $post_object->post_type )
        return;

    // verify this came from the our screen and with proper authorization,
    // because save_post can be triggered at other times
    if ( !wp_verify_nonce( $_POST['noncename_wpse_81739'], plugin_basename( __FILE__ ) ) )
        return;

    // Check permissions
    if ( 'page' == $_POST['post_type'] ) 
    {
        if ( !current_user_can( 'edit_page', $post_id ) )
            return;
    }
    else
    {
        if ( !current_user_can( 'edit_post', $post_id ) )
            return;
    }

    // OK, we're authenticated: we need to find and save the data

    //sanitize user input
    $mydata = esc_textarea( $_POST['new_field_wpse_81739'] );

    $time = current_time('mysql');
    global $current_user;
    get_currentuserinfo();

    $data = array(
        'comment_post_ID' => $post_id,
        'comment_author' => $current_user->user_login,
        'comment_author_email' => $current_user->user_email,
        'comment_author_url' => 'http://',
        'comment_content' => $mydata,
        'comment_type' => '',
        'comment_parent' => 0,
        'user_id' => $current_user->ID,
        'comment_date' => $time,
        'comment_approved' => 1,
    );

    wp_insert_comment( $data );
}

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