h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Keith Richards of the "Stones" said "we're all a big
gang of beggars and thieves" in reference to rock musicians
"stealing" licks/riffs etc. from each other. It's true!

In classical music also there is an extensive history of plagiarism. In the renaissance the use of themes written by another composer was considered a compliment. This practice continued through the baroque period and into the early classical period.

JS Bach was particularly adept at plagiarising himself. The B-minor mass has substantial sections lifted from his previous pieces. I don’t know if Mr. Bach was consciously complimenting himself or not.