fatal error

on this page:
http://www.groupomater.com/store/

i receive:

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

any thoughts on how to fix it?
thanks
JB

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