I concede that repetition is a valuable part of learning and that there's no easy answer, but they way Duolingo does it seems pretty intentional given the level of polish they have in the app generally. When prior lessons reset, it can interrupt your progress on your current lesson and will actively block you from making further progress until you go back and do those lessons again, unless you "skip" them, which they use their weird sad owl to discourage. Often the lessons haven't changed very much and redoing it is just busywork, seeing the first couple words of a question you've seen before and remembering what the answer is before its even finished writing it out, which doesn't seem like it reinforces learning the language, just learning how to salivate when Duo rings the bell.

I haven't used Anki either and I'm not suggesting anybody ban either one, though I would be curious how Anki's spaced repetition implementation differs from Duos. In general I don't think bans usually have the intended effect, and trying to ban or discourage dark user patterns seems almost impossible to define usefully, let alone enforce even if it were the right thing to do. I'm not much of a gambler, but I live in one of the holdout US states that doesn't allow sports betting apps, which is an entire ecosystem of human-hacking dark patterns, and it bothers me that I'm disallowed from participating in the name of protecting other people from their own poor self-control. Duolingo has all the time in the world to defeat that kind of thing with loopholes and it would make it even harder for an alternative service to compete with them.