WordPress RSS Feed does not validate

I have been submitting the RSS Feeds from my WP websites and every time I submit them to the RSS aggregators, the feed does not validate. I have been getting the following error message:

guid values must not be duplicated within a feed: http://monokiniswimwearsale.com/%/postname%/

<guid isPermaLink=”false”>http://monokiniswimwearsale.com/%/postname%/</guid&gt;

Any ideas as to what is wrong and how to fix it?