James -- An interesting take on the subject indeed, but I don't think it's exactly the same issue. I'm not concerned about writing a song with the same subject matter or even the same treatment as a pre-existing song; my concern is with unconsciously copying another writer's melody, one which I've heard at some point in the past, perhaps while not even consciously aware of it at the time, and then having that melody resurface in my mind while I'm composing what I think is a work of my own.
Along those lines, has anyone used any of the melody search engines, such as midomi, WatZatSong, or musipedia to screen your melodies for possible duplication of another's work and, if so, what was your opinion of their utility? In my experience, they seem to generate a lot of false positives.
What I'm trying to do with this topic is find a way to avoid accidentally infringing on someone else's copyrighted work.