Duolingo (NASDAQ:DUOL) is a growth stock that has been on a stomach-churning round trip since its IPO in 2021. I bought shares of the language learning firm three times in 2025. I feared this was just a buzzy, gamified language learning app that could easily be replicated. However, one by one, it started ticking off boxes on my growth stock checklist. Its AI model is trained on billions of daily learning events.