I think duolingo should add a dictionary so you could look at words you've already learned to refresh your mind IN THE APP
How to see the hidden 'Words' tab?
The 'Words' tab is hidden in some courses.
If you want to view it, here is a little trick:
1) Go to a course with a 'Words' tab (French for example, otherwise only one of the following will work: English, German, Italian, Portuguese, Spanish).
2) Open a new tab (right click on the tab, then 'New tab' in your (e.g. Chrome) web browser), then go to http://www.duolingo.com and switch to a language course without the 'Words' tab (e.g. Polish).
3) Go back to the first tab in your browser (French) and click on the 'Words' tab (do not reload the page).
4) You should see the word list of the Polish course or any other course you switched in the step 2.
5) This trick is not persistent, when you want to use the "Words" tab you have to follow this steps every time.
See: also: https://www.duolingo.com/comment/21034166