The more I exercise on Duolingo, the more weird advertisement I meet on it.
Sometimes I let little children play Duolingo for learning languages, I don't want them to see this so early :/
I believe that the mistake is not on Duolingo's part. The advertisement is randomly selected based on your browsing behavior by Google. I my case, I always get the World of Warship ads, because I search a lot on gaming. But I can be wrong.