A prominent Wall Street investment bank just upgraded the stock to buy.After tumbling from a peak it set in May, shares of Duolingo (DUOL 2.95%) recently received a positive bump from a Wall Street investment bank that follows its business closely.Duolingo stock popped about 4% higher when the market opened on Friday, July 26. Investors were responding to encouraging words from Bank of America analyst Curtis Nagle. Citing a 31% price drop following the company's first-quarter earnings report in May, Nagle u ...