I know what you mean Jed. Every time I write a song I'm scared to sing it to anyone for the first time in case I've unknowingly lifted the tune.

What I do sometimes, is deliberately start with a known tune and then modify it until it is hopefully unrecognizable. No one has yet told me that the tune for Bathing Angel resembles the Eton Boat Song although that's where it started.

Then there are those songs that use a tune that you had written for an earlier song and the tune refuses to be changed because it fits the new song better than it did the old one.