Duplicate RSS items

I have recently begun using the RSS capability in Thunderbird. I am using it to subscribe to verious cateories of Google news. The problem that I'm experiencing occurs when TB checks for new rss items. It finds many of the stories I already have, but instead of realizing that i ialready have them, TB creates new duplicate stories. As you can imagine, I am quickly piling up many, many copies of each story. I have changed my update rate from 2 minutes to 60, but this doesn't totally solve the problem. Isn't there a way to only have one copy of each story? I've used the rss capability of the Google sidebar before, and I never experienced this problem. (BTW, I would love to have a portable version of the Google sidebar!)

Thanks in advance!