Duolingo’s ( DUOL ) second quarter was the sort of report that sharpens a thesis. The company lifted revenue 41% year over year to $252.3 million. It also lifted adjusted EBITDA margin by about four points to 31.2%, andI am a retired quant with a PhD in mechanical engineering. I started off my professional career as an engineer and eventually transitioned into a hybrid developer/quantative analyst role at the investment arm of one of the nation's largest insurance companies.I ended my career as a fixed inco ...