It is possible, though I think it’s one of those products whose success is based more on customer testimonials than actual statistics about it’s effectiveness.

They might exist, but I haven’t met anyone who has said they were able to use duolingo to become fluent or even competent in a language.

But then again, my German learned from a class in high school isn’t much better. Hell, my French leaned from being in French immersion all through elementary school followed by normal French classes in high school isn’t even at a competent level, though I can at least communicate a bit in French. I can still see those subject-verb conjugation tables though lol (though I’ve lost the French version of “them/they”).