In Eunuchus by the Roman playwright Terence, there is a prologue penned for the explicit purpose of justifying the author's reuse of existing characters. In a section, the author identifies some common literary tropes of his time and defends his reuse on those grounds:
When the officials were present, a run-through began. He cried out that it was a thief, not an author, who had put the play on, but that he hadn’t put one over on him all the same. He said that there was a play called The Toady by Naevius and Plautus, an old play, and that the characters of the parasite and the soldier had been lifted from it. If that wrong has been done, it was done through the author’s ignorance, not because he was keen to commit a theft. That this is so, you will now be able to judge. There’s a play called The Toady by Menander; in it there’s a parasite (the Toady) and a boastful soldier. The author does not deny that he has transferred those characters to his Eunuch, from the Greek play; but he does utterly deny that he knew those plays had been written previously in Latin. And if he’s not allowed to use the same characters as someone else has used, how is it more allowable to show a running slave, to make his mothers good and his prostitutes bad, [a parasite gluttonous and a soldier boastful,] to write about a supposititious baby, the deception of an old man by a slave, love, hate, suspicion? In short, there’s nothing said today that has not been said before. So it’s right that you should acknowledge and allow it, if new writers do what old ones used to do.
From the Peter Brown translation
This seems very similar to the justification behind the modern legal doctrine of fair use. Is this the earliest example of such a defense, or is there an earlier instance of defending recycling intellectual property in a literary context?