what counts as "new" in a feed?

[Carrot Wax]

I recently noticed some bad URL and SEO terms in the page that was in
one of my feeds, so I fixed it. The problem was that Feedburner now
thought this was an entirely new item in the feed and sent it out to
all the subscribers. They were understandably annoyed because it was
a duplicate. Some people unsubscribed.

I even noticed that it showed up in the RSS feed before it got sent
out to the email subscribers but could find no way to tell Feedburner
"No, this isn't really new, please don't send it out".

So questions:

1) What causes Feedburner to mark something as "new"? If I change
the title? Words in the text? URL? SEO terms?

2) Is there a way to tell Feedburner that this isn't new, and not to
send it out?