Rated 3 out of 5 stars
The Alt+Enter on the url doesn't actually duplicate the tab completely, just the current url with GET parameters.
If you got to the current page via a POST form (eg. one that does something you wouldn't want to repeat or has a permanent effect), or want to keep the history for the duplicated tab in the new window, Alt+enter doesn't help you.
How much of the state does this extension duplicate? I'm going with Tab mix plus instead... (sometimes I use Opera browser too, which has a lovely duplicate tab function you can bind to any key/menu that keeps everything about the tab)