Update prevent duplicate purchases

[mihaijoldis]

Based on an .org ticket https://wordpress.org/support/topic/plugin-code-from-edd-library-does-not-work/ we should update our snippet to allow Renewals and Upgrades if SL is being used. https://library.easydigitaldownloads.com/checkout/prevent-duplicate-purchases.html

Working code (updated) from @robincornett

function pw_edd_prevent_duplicate_purchase( $valid_data, $posted ) {

	$cart_contents = edd_get_cart_contents();
	foreach ( $cart_contents as $item ) {
		if ( ! empty( $item['options']['is_upgrade'] ) ) {
			continue;
		}
		if ( ! empty( $item['options']['is_renewal'] ) ) {
			continue;
		}
		if ( edd_has_user_purchased( get_current_user_id(), $item['id'] ) ) {
			edd_set_error( 'duplicate_item', 'You have already purchased this item so may not purchase it again. If you want to upgrade, click on My Licenses and select the next upgrade level!' );
		}
	}
}
add_action( 'edd_checkout_error_checks', 'pw_edd_prevent_duplicate_purchase', 10, 2 );