add_filter( 'post_row_actions', 'rd_duplicate_post_link', 10, 2 );
add_filter( 'page_row_actions', 'rd_duplicate_post_link', 10, 2 );
function rd_duplicate_post_link( $actions, $post ) {

    if( ! current_user_can( 'edit_posts' ) ) {
        return $actions;
    }

    $url = wp_nonce_url(
        add_query_arg(
            array(
                'action' => 'rd_duplicate_post_as_draft',
                'post' => $post->ID,
            ),
            'admin.php'
        ),
        basename(__FILE__),
        'duplicate_nonce'
    );

    $actions[ 'duplicate' ] = '<a href="' . $url . '" title="Duplicate this item" rel="permalink">Duplicate</a>';

    return $actions;
}

add_action( 'admin_action_rd_duplicate_post_as_draft', 'rd_duplicate_post_as_draft' );

function rd_duplicate_post_as_draft(){

    // check if post ID has been provided and action
    if ( empty( $_GET[ 'post' ] ) ) {
        wp_die( 'No post to duplicate has been provided!' );
    }

    // Nonce verification
    if ( ! isset( $_GET[ 'duplicate_nonce' ] ) || ! wp_verify_nonce( $_GET[ 'duplicate_nonce' ], basename( __FILE__ ) ) ) {
        return;
    }

    // Get the original post id
    $post_id = absint( $_GET[ 'post' ] );

    // And all the original post data then
    $post = get_post( $post_id );

    /*
     * if you don't want current user to be the new post author,
     * then change next couple of lines to this: $new_post_author = $post->post_author;
     */
    $current_user = wp_get_current_user();
    $new_post_author = $current_user->ID;

    // if post data exists (I am sure it is, but just in a case), create the post duplicate
    if ( $post ) {

        // new post data array
        $args = array(
            'comment_status' => $post->comment_status,
            'ping_status'    => $post->ping_status,
            'post_author'    => $new_post_author,
            'post_content'   => $post->post_content,
            'post_excerpt'   => $post->post_excerpt,
            'post_name'      => $post->post_name,
            'post_parent'    => $post->post_parent,
            'post_password'  => $post->post_password,
            'post_status'    => 'draft',
            'post_title'     => $post->post_title,
            'post_type'      => $post->post_type,
            'to_ping'        => $post->to_ping,
            'menu_order'     => $post->menu_order
        );
        $new_post_id = wp_insert_post( $args );
        $taxonomies = get_object_taxonomies( get_post_type( $post ) ); // returns array of taxonomy names for post type, ex array("category", "post_tag");
        if( $taxonomies ) {
            foreach ( $taxonomies as $taxonomy ) {
                $post_terms = wp_get_object_terms( $post_id, $taxonomy, array( 'fields' => 'slugs' ) );
                wp_set_object_terms( $new_post_id, $post_terms, $taxonomy, false );
            }
        }

        // duplicate all post meta
        $post_meta = get_post_meta( $post_id );
        if( $post_meta ) {

            foreach ( $post_meta as $meta_key => $meta_values ) {

                if( '_wp_old_slug' == $meta_key ) { // do nothing for this meta key
                    continue;
                }

                foreach ( $meta_values as $meta_value ) {
                    add_post_meta( $new_post_id, $meta_key, $meta_value );
                }
            }
        }

        wp_safe_redirect(
            add_query_arg(
                array(
                    'post_type' => ( 'post' !== get_post_type( $post ) ? get_post_type( $post ) : false ),
                    'saved' => 'post_duplication_created' // just a custom slug here
                ),
                admin_url( 'edit.php' )
            )
        );
        exit;

    } else {
        wp_die( 'Post creation failed, could not find original post.' );
    }

}

add_action( 'admin_notices', 'rudr_duplication_admin_notice' );

function rudr_duplication_admin_notice() {

    // Get the current screen
    $screen = get_current_screen();

    if ( 'edit' !== $screen->base ) {
        return;
    }

    //Checks if settings updated
    if ( isset( $_GET[ 'saved' ] ) && 'post_duplication_created' == $_GET[ 'saved' ] ) {

         echo '<div class="notice notice-success is-dismissible"><p>Post copy created.</p></div>';
         
    }
}