Some certificate that you could show off- that requires ID and it totally legit
It would be useful to show how good you got on Duolingo- a certificate that requires ID and you can show to friends and employers- you did a course in duolingo...
As fun as that would be, I think that it would be way too hard for Duolingo to program some way of checking your ID or something similar, and it would be a lot of hassle for them and for you to deal with verifying that it is in fact the same person on the ID writing the test (a proctor would probably be required). I just think that it would be too much work and hassle when you could just go somewhere and take a well-known standardized test and show off your mark in it as opposed to trying to get legitimacy that it was in fact you who learned some of a language online, especially since the only reasonable way Duolingo has of measuring language proficiency in a quantifiable manner is the 25 Lingot test, which is hardly any sort of well-known one so it really doesn't matter if you prove that it was you who did it.
You can always just put it on your resume that speak [language]. It's not professional qualification but it should be taken pretty seriously. Lying on your resume could get you in big trouble so people would probably assume that you wouldn't have put it on there if you couldn't speak it.