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------------

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