The haveItem check for RSS and Atom causes the feeds to act unexpectedly.
For RSS, the checked tags don't necessarily have to be unique. For atom,
it is allowed to have duplicate items (including duplicated ids) in one feed,
so this shouldn't be stopped either.
In RSS feeds, the author of an item was always overwritten by a non-standard
creator tag. Change this so that creator is only used when it actually
appears. Otherwise use the previous value of Author, whatever that is.