Gamified mobile learning company Duolingo (DUOL 1.35%) has been a market-crushing investment. The stock closed its first day of trading in 2021 at $139 per share. As of this writing, just four years later, it's trading at over$520 per share, up 276% and trading near an all-time high. For perspective, the S&P 500 is up just 34% during this time.Duolingo is outrunning the market for at least two simple reasons. First, it's growing like a weed. Second, it's actually quite a profitable business. The end result ...