Error when visiting store

I recently installed this plugin and sometimes the store comes up fine, sometimes it comes up with this error:-

Fatal error: Cannot redeclare querystring() (previously declared in /home/robbrown/public_html/therobbrown.com/wp-content/plugins/simple-e-commerce-shopping-cart/views/store-home.php:2) in /home/robbrown/public_html/therobbrown.com/wp-content/plugins/simple-e-commerce-shopping-cart/views/store-home.php on line 9

Can anyone help me fix this?

http://wordpress.org/plugins/simple-e-commerce-shopping-cart/