Thank you, that is not the answer I was hoping for, but, unfortunately it is the one I expected.
Just so I am clear - the reason this is happinging is because I am posting to index.cgi and index.cgi is printing out a web page. To stop it from happening, I would need to post to index.cgi and have it redirect to a web page of some kind?
Another big question is why does this only happen with one client - I have personally never seen it and the majority of the other people I have using the site have only sporatically seen it.