I've installed the duo plug-in but, when I activate it the site (front-end as well as admin area) fails to load and I get the following error message in my logs:
PHP Fatal error: Call to undefined function get_current_site() in /web/.../wp-content/plugins/duo-wordpress/duo_wordpress.php on line 722
If I copy the definition for the get_current_site function from ms-functions.php into duo_wordpress.php, the pages will load and I can configure duo, but that is not ideal for longterm maintenance. Also, at this stage of testing, I am unsure what other consequences there might be.
My site is not a multi-site installation and so is not loading the multi-site functions by default.