"Das muß ich selbst auch immer wieder tun."
Yeah, I think the problem here is that before the German Spelling Reform of 1996 "muß" would have been correct here, and I sometimes get the feeling that some of the people that put together the Duolingo German lessons are loath to make the switch. But now, in German schools anyway (so I hear), they teach "muss" instead, and, for that reason, I think Duolingo should also be teaching "muss". I've complained about this one before via the "feedback" link, but obviously I must have been ignored if the sentence is still written as shown above.
I first studied German in the early 80s, and back then "muß" reigned supreme. And I have to say, I kind of miss the German eszett here, but that's how it is. The 1996 Spelling Reform is an interesting topic all to itself. If you care, Wikipedia has a good summation.
Yup--I, too learned German before 1996. It's interesting--the "new" rules are far less complicated (ss after short vowels, eszett after long). But the old rules meant you could always use ß at the end of a syllable, which meant that, as a non-native speaker, you didn't have to know whether the vowel was short or long to spell correctly! (Often it's obvious, but sometimes it's tricky, especially for vowels where the color doesn't change between short & long,)