There's a huge problem with recycling translations. Loads of words and phrases have different meanings in different contexts. Also, you have to take into account semantic differences between local dialects. For instance, the term "public school" means something completely different in British and American English. In the US, it's a state-funded school, whereas in the UK it's an expensive private (boarding) school. Now, if you have an American and a British text talking about public schools, obviously, the correct translations into the target language will be completely different. Duolingo, however, will inevitably produce an incorrect translation at this point. You definitely need some kind of manual review before you lock a recycled translation.