Question: just 1 click ..and it duplicates ALL content, settings, css, plugins..

im looking for a plugin that adds a new link to Sites admin; 

Edit | Dashboard | Deactivate | Archive | Spam | Delete | Visit | **DUPLICATE**... 

it should copy ALL settings/configs. 
and create a new subdomain instance in the **existing** Multisite

Answer: 
A quick edit option for |Delete|Visit|DUPLICATE seems a bit too tailor made.

Nonetheless, this plugin does exactly what you want with 2 clicks (on its own screen).
<http://wordpress.org/extend/plugins/add-cloned-sites-for-wpmu-batch/>
Late to the party here but wanted to mention for the benefit of future searchers that we had the same need and built a plugin for cloning/duplicating with all settings intact just like the OP describes.

It is called the [NS Cloner](http://wordpress.org/plugins/ns-cloner-site-copier), and it doesn't add a "Duplicate" action link as described here, but it makes the process almost that easy by creating an easy-to-use network admin page where you can select a source site and then create a perfect duplicate complete with all content, plugin settings, etc, and it tops the WPMUDev New Blog Template plugin by being free (their are some more advanced features you can add with add-ons but the very powerful core is forever free), adding more power/flexibility in cloning settings, and avoiding hangups with updates since it's a wordpress.org plugin.