I just got hold of a copy of "La Settima Donna" by Roberto Pregadio, and the 3rd track on the CD "La Spogliarello" is basically "I Feel Love" by Donna Summer.
I remember thinking that "You Don't Care" by Cricket (Aurelio Grimaldi / Fabio Frizzi) (from "Amanti Miei") was pretty much "Le Freak" by Chic (though it's also close to the theme from "Manaos", credited to Fabio Frizzi, which sounds less like "Le Freak").
"Epopea" by Il Reale Impero Brittanico and from the film "Perché si Uccidono" is basically "Theme One" by George Martin (and which I knew as a single by Van der Graaf Generator).
I'm guessing this probably happened because the films were temp-tracked and the composers were told to keep very close to the music used to do that. What do I know, though? Does anyone know any more about why this happens? Do you have really notable examples you'd like to share? I find it really jarring when it happens to the degree it does in the examples above. Apologies if this idea has been done to death (I did search a bit).