I and my clients have some users who love to track their progress through multiple completion and others, that prefer to have the visual guidance of the checkmarks on the lesson overview to mark their progress and easily find todays lesson. Right now we can only favor one type of users, so I would suggest adding the option to mark uncomplete to the multiple completions option, just a wee text below the mark complete button to uncomplete instead, it could even still track the previous completion.

And we also really need the remove all completions button requested in several other posts.