Basically, when you finish a course, you might want to study it again to really understand and lock the information in. But when you already have everything as mastered (I am very much a perfectionist when it comes to Khan Academy courses), it becomes less motivating to continue or even (re)start, or you lose where you were, or if you make two mistakes on one quiz, then you not only have to fix it, but also take the Unit Test again. I suppose that could be different for different people, as I do want every course of mine to look really nice.
My suggestion is that for one course, we can make new attempts. Your first attempt could be in any shape, and you add another attempt to have a clean slate to start over again, while still keeping the first attempt to maybe flex (if it was really perfect or something); but I guess maybe add a feature so that you could also delete attempts too. This way you don't have to make a new account just for that one course, or delete your old attempts for each quiz (especially if you worked really hard for it).
Thanks for reading!