I too have noted an erie similarity of many of the songs to classical works that pre-date the CD by several hundred years.
Yes, I've noticed that too. Not so much in the melody, but in the harmonies and chord sequences. You can sometimes even pick the composer it was borrowed from. I'm learning some Bach pieces at the moment, and occasionally I'll be playing a new passage and think "Dang, where have I heard that before...?"
There was even an amusing misunderstanding in 1933 when German JWs were accused of supporting the Nazis after opening a convention with the German national anthem. It turned out that the song was "Zion's Glorious Hope", and both songs had been based on the same work by Joseph Haydn. Well that's their story anyway, and they're sticking to it...