It's also in the wrong place in the menu - it would usually be just after Paste. Try Window>Workspace>Update and Merge Menus - although I don't think the command wa in that menu initially, and I'm not certain it was in 4.6 at all. If you can't find the command, or if it doesn't work, try reloading your layout - though I am not certain that Duplicate itself would be there in 4.6, I don't think it was always present.