Last year many readers gnashed their teeth when they got to the epilogue of the last HP book, which the author used as a way to constrain post-copyright abuse and writers of fan fiction. Why she cared, I don't understand, since she could so easily distract herself by buying an island or something. Maybe it was an act of self-discipline, to remove the temptation of writing more of the same thing.
Anyway, there is a canonical precedent. Part 2 of Don Quixote (1615) was published 10 years after Part 1 (1605). At first, Cervantes has great fun with the idea that everyone Quixote and Sancho Panza meets already knows them, having, of course, read Part 1, a smash bestseller. But then, while in the middle of writing the novel, Cervantes (the actual Cervantes) come across a continuation of Don Quixote, published in 1614, and the fun turns sour. Cervantes is furious.
In Chapter 59, Don Quixote (the character) comes across the faux Don Quixote (the book) in an inn. Don Quixote (the "real" character) is on his way to the tournament in Saragossa, but it turns out that the "fictional" Don Quixote goes to Saragossa. So:
"Don Juan informed him that this new history told how Don Quixote, whoever he might be, in that same tournament had participated in a tilting at the ring but that the description given had shown a sorry lack of inventiveness, especially with regard to the mottoes of the knights and their liveries, in which regard it was impoverished in the extreme though rich in foolishness.
'For that very reason,' said Don Quixote, 'I will not set foot in Saragossa but will let the world see how this new historian lies, by showing people that I am not the Don Quixote of whom he is speaking.'"
So they go to Barcelona instead. The false Don Quixote keeps turning up.* The "real" Don Quixote visits a notary, to get a sworn statement that he is the real Don Quixote. And then there's more notarizing at the end:
"Perceiving that their friend was no more, the curate asked the notary to be a witness to the fact that Alonso Quijano the Good, commonly known as Don Quixote, was truly dead, this being necessary in order that some author other than Cid Hamete Bengali might not have the opportunity of falsely resurrecting him and writing endless histories of his exploits." (Ch. 74)
This is why people talk about Don Quixote as the first postmodern novel, this and the "Cid Hamete Bengali" business.
Dickens fought a similar problem most of his life. His serialized novels took 18 months or so to publish. Theatrical versions, with their own endings, would appear before he was done. Nicholas Nickleby has an ill-judged chapter where Nicholas rants about this evil, targeting a specific hack writer. Dickens would write his own "official" theatrical versions which would be rushed into production a few days after the last installment of the serial appeared. Notarization did not help; enforcement of copyright law did.
* The false Don Quixote, but never, per Nabokov's suggestion, the false Don Quixote. Nabokov wanted the "real" and "false" Don Quixotes to joust.