Why not have learners evaluate the quality of Duolingo's lessons along several criteria so you know whether to attempt a lesson or wait until it gets better?
You could also have email alerts for lesson quality. So for example, you might set up an email alert to tell you about lessons that have now reached certain levels of quality along several criteria that you care about (e.g., how flexible your answers can be, how interesting the sentences are, etc.).
Duolingo lessons could be treated somewhat like iOS apps with respect to updates, update descriptions, user ratings for an update and overall, and reviews for each update.