There was a pretty good example last month. I don't think any musician listening to the two songs would say one is copied from the other. Maybe inspired by, but not copied. Nearly every march has horns playing upbeats, a trio section, and a stinger at the end. That doesn't mean the estate of John Phillip Sousa should be suing every composer of marches who came after him.
But even when you are "copying" the nature of copyright law still stifles innovation of transformation. I arrange music for a wind ensemble at my church. In order to even do that, I have to get written permission from the original copyright holder, which isn't trivial in most cases. That's time I could be writing music instead of doing paperwork. Yes, I could write my own tunes, but a congregation isn't going to connect with original music like they do with something they've heard 50 times before.
Then you've got companies who make a business model out of publishing public domain works, copyrighting the edition, and then suing anybody else who tries to publish the same public domain work. Not just music either, but books and photographs, too. Granted, this isn't a rampant problem, but it's enough of a concern/annoyance that some people drop out of creative markets altogether.