item should contain a guid element

From the RSS 2.0 Specification:

It's recommended that you provide the guid, and if possible make it a permalink. This enables aggregators to not repeat items, even if there have been editing changes.

A frequently asked question about <guid>s is how do they compare to <link>s. Aren't they the same thing? Yes, in some content systems, and no in others. In some systems, <link> is a permalink to a weblog item. However, in other systems, each <item> is a synopsis of a longer article, <link> points to the article, and <guid> is the permalink to the weblog entry.

Additional useful information can be found here:

• RSS Duplicate Detection

Add a different, unique, and unchanging guid to each item. See scripting.com for an example.