Based on multiple sources (non-anthology)

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https://www.criticker.com/films/?filter=e44210

As in distinct sources 'combined' into a narrative feature. Not including anthologies like O. Henry's Full House, but including instances of multiple sources of varying length, like The Long, Hot Summer, which uses pieces of a short story, a novella, and a novel by Faulkner.

I'd be interested to learn of more of these, especially outside North America

Late Chrysanthemums (Fumiko Hayashi)
The Long, Hot Summer (William Faulkner)
Morning Patrol (Daphne du Maurier, Philip K. Dick, Raymond Chandler, Herman Raucher)
Bright Angel (Richard Ford)
Short Cuts (Raymond Carver)
A Far Off Place (Laurens Van der Post)
The Grand Budapest Hotel (Stefan Zweig)
Certain Women (Maile Meloy)
Julieta (Alice Munro)

(more in collection)

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Came across an interesting instance I didn't know about--About Schmidt:

Payne's script to About Schmidt was initially an original screenplay written years before Begley's novel was published. According to Payne, his script was about "an old guy who retires, and realizes how much he’s wasted his life, and wants somehow to start anew— The Graduate at age sixty-five." Payne completed the script in 1991 and offered it to Universal Pictures, but the studio rejected it. Following the publication of Begley's novel in 1996, Payne decided to combine his script with the plot of the novel, thus making it an adaptation.

I'm a little hesitant since the only thing distinguishing it from simply a loose adaptation is publication order, but then that's the thing--the act of amalgamation is still much the same.