"The husband has a wife."
Translation:Maritus uxorem habet.
'wife' is the object and so takes the accusative case. This link might be helpful to you: https://classics.osu.edu/Undergraduate-Studies/Latin-Program/Grammar/Cases/latin-case
"Marito uxor est" is a more archaic way of expressing possession, originally most likely inalienable: [DAT = POSS'ER] + [[NOM = POSS'ED] + ESSE]
You could also use it to say "a husband has a wife at home" = Marito uxor est domi. But you can't say it is "more Latin-like" (latinius) than "Maritus uxorem habet," since both structures are equally 'latine' from what I can tell: http://www.perseus.tufts.edu/hopper/text?doc=habeo&fromdoc=Perseus%3Atext%3A1999.04.0059
No offense intended, but it's a bit a stupid sentence. I can't find a situation where an husband has no wife. It's the definition in itself...
It's called a pleonasm, or a redundancy and normally, it's not correct. We should learn to avoid them instead of using them. It hurts the logics.
You're absolutely right. But the insertion of this sentence into the lesson (and others like it, such as 'Vir maritum habet', the man has a husband) is less a problem for its pleonasm than for its presentism, the centering of present-day values where they don't belong. You may very well think it's obvious that a husband would have a wife, but the purpose of this mischief is to problematise things that are normative, and to normalise things that are inherently non-normative. The educational and media caste of the present-day West does it all the time.
Duolingo is certainly quick to promote the liberal agenda, without even caring that children might use this service. In the other hand, they leave out important and very commonly used expressions that has any mention to God, Christ or Christianity in general. That's why I'll never acquire Duolingo Plus.