Duolingo is utterly useless! The only thing it is good for is pretending you are learning a language and sucjing you into a cycle of logging into the app and pretending you are learning something of merit. It doesn't teach you any practical knowledge about the language or how to use it it just throws vocab at you with no explanation of how to use it properly.
I feel pretty strongly about this because I absolutely wasted over a year and a half logging in daily, doing their shitty exercises and thinking I was learning things, when the fact of the matter is all you will learn is some random phrases and words but with no real basis on how to use them and the language properly.
I too am also learning Spanish as my gf is Spanish and she told me multiple times as well that what Duolingo was teaching me was flat out wrong, so not only is it a shit teacher it was teaching me incorrect things that I then had to try and unlearn.
It is fucking trash!
A better app if you want that style of learning is Busuu, that actually explains concepts properly and tries to teach you how to use the language correctly. In one month on there I felt like I'd learnt more useful information that in all the time I wasted on Duolingo.
Even better than that (and mentioned by another commenter) is language transfer. This is all free on SoundCloud to listen to and will teach you how to use your own language as a benefit and how to work out the Spanish from your existing knowledge. It doesn't try to force you to memorise inane and pointless vocabulary it tries to teach methods and ways to actually learn the language for practical real world use. Listening to this was like an aha moment for me as lots of different things suddenly started to make more sense.
In short, fuck Duolingo, if you want to pretend you are learning a language then feel free to waste your time on there but if you actually want to use it with the people you work with then use something else.