I've noticed this phenomenon occur in a few movies and wanted to document it. I'm certain there's some analysis on the matter but I hope this list might be useful for the purpose of identifying cases. Entries will have spoilers insofar as the scene in which a composer's reused track(s) is/are used will be described.
I should note that movies in which composers carry over tracks/themes from previous films in a franchise they composed will not be listed (ex. John Williams reusing themes from previous Star Wars movies for subsequent movies in the franchise), as the trend I'm trying to draw attention to is the use of preexisting music from a different piece of work/media by a composer for an entirely unrelated film.
If you have any recommendations for additions I would greatly appreciate it.