I don't remember if this has been mentioned, but I've seen this off on and for at least a month or so now. Every once in awhile (pretty low frequency) I'll get the exact same word/character twice in a row, the stats for the second time don't register that I just completed it. It seems to happen more when I am adding new words (using the + button) but doesn't necessarily happen with the words that are new. The most recent word that it happened for me on is: 棉毛衫.
Firefox 3.6, windows xp