Someone online said, "You can't parody a parody." Do you agree or disagree with this claim? Why?

Strongly Disagree. Regardless of how its sold, any work that seeks to express stands open to interpretations that may defy what the sales pitch suggests.
"Disaster Movie" may suggest itself as a parody, but your interpretations may see it as a bunch of references here and there with the only rhyme and reason that "it exists!". Instead of ridiculing movies, you see them parasitizing their fame, in hope of scoring jokes that'll win the audience. Its not a parody, but a parasite. So in retaliation, you make "It Exists: the movie!"
With this we get that the genre, as a product, defines nothing. Its merely the voice of any given entity (the author, a group of marketers, etc) that tries to convince us that their work is *this* because *insert reason*.
So, prohibiting any expressive work because of a series of sales pitches that have happened in the past... I disagree.

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