I've recently started using the "add" button more and a couple of times have run into a minor problem where words will be added twice. For example, I'll be prompted to write a new word, then immediately I'll be prompted with that word again.
Maybe this has to do with clicking the button quickly in succession. Also, if I refresh the page or change to studying only some parts, the problem goes away. So it's not a huge problem, just a minor annoyance.
James