PHP & RSS

I have been using the OnyxRSS PHP parser and am really happy with it.

Of course when I open the web page to check on news, there is a chance
I will see the same stories again if I am opening the page inside of
the news site's refresh window.

How do Aggregators such as Awasu and FeedDemon get round this, are they
storing news subject headings in a database and checking against that
before determining whether to display a news item as new?

If so is that a function of the RSS spec that has some hooks on how to
implement or should I just start chucking subject headings into a MySQL
database and searching that before displaying?
--
Stuart
(take out the trash if replying by mail)