I'm getting an error I want you to fix.

I just installed the dashboard plugin you gave me, but when i try and install the marketpress module i get the following error:

Fatal error: Cannot redeclare class MarketPress_Shopping_Cart in /home/webelie/public_html/powerplantNRG.com/wp-content/plugins/marketpress/marketpress.php on line 7569

Please help.
Thank you for your time,
John Baxter