This clue from the last earnings report is signaling a buying opportunity.Duolingo (DUOL +1.68%) stock is down 28% since the company reported third-quarter financial results on Nov. 5, despite exceeding expectations for revenue and subscribers. The stock fell due to soft guidance for the fourth quarter, when the company expects significantly lower growth in bookings.However, growth in revenue or bookings doesn't tell investors about the real health of the business, which is how well Duolingo is retaining it ...