Duolingo Inc (NASDAQ:DUOL) stock is surging today, after news that the mobile learning platform will join the S&P MidCap 400 Index, effective April 22. At last glance, DUOL was up 7.2% at $208.64, earlier as high as$217.79. The 20-day moving average seems to be keeping today's gains in check, though the stock is above a host of long-term moving averages, including the 120-day trendline. Today's bounce could've already been in the cards, per Duolingo stock's 14-day relative strength index (RSI) of 26, which ...