Report a problem, problem.
In Spanish, I had to translate the phrase: "De quién es el caballo?" (I think), to which the answer is "Whose horse is it?".
My answer was "Who's horse is it?", which was accepted as a typo. This is not a typo and should absolutely not be accepted as correct.
I could not report this as a problem. When I clicked on report a problem, I expected/needed to see "My answer is not correct" (much like in the reverse scenario when your answer is not accepted but you believe it should be and clicking report a problem reveals the option "My answer is correct" or words to that effect).
I posted this under Duolingo as this same situation could, I guess, occur whilst learning any language. I just happened to be learning Spanish.
There is another way you might not have noticed to report a bug or an error in the database such as what you describe. I know i hadn't noticed it for months... There is a "feedback" button on the left of the screen. It is less ideal than reporting an error after a question, since it won't point the duolingo team to the specific question directly, but it does point them to the lesson at least. And you get to explain the problem at length, too.
Someone from duolingo will probably end up reading a thread like this one, but my guess is that it won't be the same people who deal with correcting the databases daily...