Works well - the option to delete newer duplicate tabs goes a long way to not losing anything you're looking at or working on.
Would be cool if there could be a colour, or some kind of alert or badge on the actual dupliacte tab itself.
A one-click, auto-appearing 'close' button would be good too, instead of having to dive into the menu.

I find the 'close all' button and section a bit unclear - both the duplicate and the original tabs are listed, with a cross by each. Does the cross close the tab, or remove it from the menu? Does the 'close all' button close all the listed tabs, or just the duplicates? It's a bit nerve-wracking at first, but once you get used to how it works it's helpful in pruning down serious browsing sessions.