Philip,
I know what you mean about tunes. Every time I write a song I think, "I've heard that tune somewhere". Then I get scared to sing it to anyone in case they recognize it.

Just proves that songs don't get written in a vacuum, they must be relevant to their environment.

Lionel Bart, one of the best songwriters of our time, is a great 'borrower' but it doesn't make him any less of a songwriter. His musical "Maggie May" wouldn't be the same without the sea shanteys.

Regarding Stephen Foster; it would be more surprising to find that he (or any other great songwriter) HADN'T borrowed anything.

Bert.