Q: in backtranslation, when we translate the target language back to the source language, doesn't it be duplication? for example, suppose we have parallel input "how are you" and "Wie geht es dir", when we back translate "wie geht es dir" it is "how are you" [YM]
A: We backtranslate only sentences taken from a monolingual corpus. Of course, it could happen that they already have been used when creating the MT system beforehand. But that is not a serious problem.