It's always the issue with music because there are only so many notes and possible harmonies within the system used for popular music. No one's going to write a pop song using a 12 tone harmonic structure like some composers have done; you've only got the octave and major and minor keys; songs are bound to sound alike.
Copyright law for music must be a nightmare. It might be more clear for sampling, but it's in the nature of music to interate and build on another's ideas. It goes against the creative process but people want to get paid. I often remember an interview with Nile Rogers saying how he was flattered that Sugar Hill Gang used a riff from Chic's Good Times, but he had to sue because they also used the strings he had recorded, and it cost him a lot of money to record that with real string players.
