add_action( 'template_redirect', function() {

	// Authenticated Users Only
	if( ! get_current_user_id() ) { return; }

	// WooCommerce Cart And Checkout Only
	if( ! function_exists( 'is_cart' ) || ! function_exists( 'is_checkout' ) ) { return; }
	if( ! is_cart() && ! is_checkout() ) { return; }

	// Get User's Previous Purchases
	$args = [
		'numberposts' => -1,
		'meta_key' => '_customer_user',
		'meta_value' => get_current_user_id(),
		'post_type' => wc_get_order_types(),
		'post_status' => array_keys( wc_get_is_paid_statuses() ),
	];
	$customer_orders = get_posts( $args );

	// No Orders Found
	if ( ! $customer_orders ) { return; }

	// Get Unique Products Within Order History
	$purch_product_ids = [];
	foreach( $customer_orders as $customer_order ) {
		$order = wc_get_order( $customer_order->ID );
		$items = $order->get_items();
		foreach( $items as $item ) {
			$purch_product_ids[] = $item->get_product_id();
		}
	}
	$purch_product_ids = array_unique( $purch_product_ids );

	// Find Conflicts With Cart Items
	$errors = [];
	foreach( WC()->cart->get_cart() as $cart_item ) {
		$cart_product_id = $cart_item['product_id'];
		if( in_array( $cart_product_id, $purch_product_ids ) ) {
			$_product = wc_get_product( $cart_item['data']->get_id() );
			$errors[] = sprintf(
				'<a href="%s">%s</a>',
				get_permalink( $cart_product_id ),
				$_product->get_title()
			);
		}
	}

	// No Conflicts
	if( ! $errors ) { return; }

	// Handle Conflicts
	$message = sprintf(
		'
			<strong>Warning:</strong>
			We found item(s) in your <a href="%s">cart</a> that you have already purchased:<br />
			%s<br />
			<a class="button" href="%s">%s</a>
		',
		wc_get_cart_url(),
		implode( ' & ', $errors ),
		wc_get_account_endpoint_url( 'orders' ),
		'Visit My Orders'
	);
	wc_add_notice( $message, 'error' );

} );