I'd guess and say HTML Improvements may of updated on the 25th March but that doesn't necessary mean the data they used is from 25th of March if that makes sense. I would say if you got the 301 redirects set there is no way there is duplicates and hopefully on the next update it should resolve itself.
One thing I have noticed is that you do not have rel="canonical" set on your pages, some and including myself class this as one of the most important tags for avoiding duplicates. This would of avoided this situation furthermore I hope you don't mind but I also found that your website is access with and without tailing slashes for example:
Notice how URL 2 has the tailing slash, sadly I've had problems with people back linking with trailing slashes and this has caused duplicates. So you could use canonical on your pages and never worry about things like this again, or alternatively if you rather not use a canonical solution then you can remove tailing slashes using the htaccess file.