cannot upgrade MarketPress: fatal error

When I try to install MP plugin to upgrade from Lite, I get this:
Fatal error: Cannot redeclare class MarketPress_Shopping_Cart in /home/content/61/8024161/html/wp-content/plugins/marketpress/marketpress.php on line 5677

// Update profile fields

function user_profile_update() {

$user_id = $_REQUEST['user_id']; ------------THIS IS LINE 5677------------

SIte is not live, so I cannot post a link to it. If you need to see it, I can temporarily reveal it.

Thanks!