Dave Rado: agree with what you say in your first paragraph, but as some have said, "there are only so many notes that you can put in so many permutations" - so inevitably some tunes will sound like each other, whether they were consciously based on a previously known tune: or written as a "new tune" with the composer having let some snippet of tune in his subconscious slip into his "new composition" - I have certainly seen the latter happen among some of my songwriting friends. The best thing they can do is to ask a friend with a broad knowledge of tunes to listen to their "new tune" before they go and inadvertently infringe copyright!