What’s the best approach to create duplicated pages with paginated posts?

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Easier way would be to use a $_GET parameter (which don’t change the autogenerated canonical address wordpress uses) and the content_pagination filter like this:

add_filter( 'content_pagination', function($pages){
    if ( in_the_loop() && (isset($_GET['onepage']) ){
        $pages = [ join( '', $pages ) ];
    }

    return $pages;
} );

Happy Coding.