If your library has access to it, I find Mango more useful than Duolingo. I've used both to work on my Japanese, and my daughter, who spent a summer living there and is majoring in the language now, has a better opinion of the usage in Mango. Duolingo tends to be more informal, which might not come into play as much in French as in Japanese, while Mango teaches the "polite" forms that would be expected in casual encounters.