This is one of those simple but very useful plugins. It’s a time saver.
With this plugin, as you have guessed already by it’s name, you can duplicate a menu to add it anywhere else on your website.
So imagine you have a really huge (or not as huge) menu on your top navigation and you would like to add the same navigation to your footer. You will be able to do that in a few clicks with this plugin, otherwise you would have to manually create the menu, add the pages (or whatever you want to add) one by one. So this plugin will save you a ton of time.
Check it out on the wordpress.org repository http://wordpress.org/plugins/duplicate-menu/
After installing it just go to Appearance -> Duplicate menu.
Duplicate your menu, and then go to Appearance -> Menus, and you will see your new menu in the list.