@lopo, nice work on the plugin lately. I noticed that on my install, going from 2.1 to 2.1.1 when the upgrade function fired I got deprecated warnings about the add_option. Of course now I can't get it to go again, but it said something like: "there was an error in the way that add_option was called - deprecated since 2.3". It referenced a line in functions.php. You might look at the deprecated option in http://codex.wordpress.org/Function_Reference/add_option - I think the add_option() in duplicate-post/duplicate-post-admin.php might need a little work.
But.... you are doing a really nice job on this plugin. I've recommended it alongside my own for quite a while and really appreciate the improvements you've made to it lately.
Have you thought about adding a do_action() that other developers could hook on to for other extended functionality with other plugins?