I'm listening to Alex North's score to the 1968 political epic The Shoes of the Fisherman. I just heard one cue, "Ceremony," that North consciously adapted from his rejected music for 2001: A Space Odyssey (it's based on his cue "The Foraging," which would've introduced the proto-men in Africa at the beginning of that film). Very similar music, much different context, but he knew it would work. And it becomes interesting in an intertextual way; I'm very familiar with North's 2001 music, thanks to the Varese Sarabande CD from 1993 (Jerry Goldsmith conducting the massive National Philharmonic Orchestra), but this is how more people -- at least, more people in the late Sixties -- knew that music. And they most likely didn't know that it was originally 2001 music.
Nice touch in the liner notes: each cue lists the number of musicians used, sometimes more than 100.