Translation:Have you heard about the man who only eats ants?
For me at least, hearing "that" used instead of "who" sounds very unnatural when referring to a person. You're definitely right, I hear it often enough, it just sounds incorrect to me. It may just be me, though. For what it's worth, my intuition isn't always correct. I've found I've learned a lot about proper irregular past in English from the course. I use a lot of the wrong forms and I wouldn't have realized hadn't Duolingo corrected me!
I have heard about him actually: http://www.newsweek.com/man-eats-ants-survive-days-without-water-outback-382918
According to English composition guidelines, one should rephrase this sentence in order not to be ambiguous. "Have you heard about the man who eats only ants?" This is what is meant here. But when you write it like Duo does, it could also mean, in fact DOES mean, that the man does nothing but eats ants--around the clock. He neither washes nor sleeps nor talks. He only eats ants. I'm not being a wise guy. When I tooks English composition at Millersville State University in PA, I would have been given a big red X over the sentence and probably an F for the composition. My question is: Is not this an equally valid rule of composition in Norwegian? Should you not instead write "Har du hørt om mannen som spiser bare mauer?"