I do not link to my friend Kabuki Katze's travel blog, Wandering Kotka, nearly as often as her art blog, and that's a shame. It is loaded with plenty of insights about Bulgaria that you probably never even knew you wanted to learn. Her most recent post there discusses chalga music, and points out that music gets recycled in new and different songs, apparently completely legitimately. As I mentioned in the comments, this phenomenon has been going on for decades - though not always legitimately. (In fact, I suspect it dates back to the creation of music itself.)
As I also noted in the comments, there is a very famous example of this in popular music. So famous, in fact, that there was a lawsuit over it.
Consider first "You Need Love" by Muddy Waters. It's a blues song from 1963 written by Willie Dixon.
Next comes "You Need Loving" by Small Faces.
This led (no pun intended) to the inspiration for a rather famous musical landmark: "Whole Lotta Love" by Led Zeppelin.
And that, ladies and gentlemen, is why Willie Dixon is listed as one of the songwriters for "Whole Lotta Love". I don't know if the Small Faces version was considered a cover or just escaped plagiarism charges by being obscure. It is, however, obviously the link between the two songs.