If user is logged in, and on the front page, redirect to /?firstName=FIRST&lastName=LAST

home-page-redirect.php

<?php

function home_page_first_last_name_redirect() {
	if ( is_front_page() && is_user_logged_in() && ! isset( $_GET['firstName'] ) ) {
		$user = wp_get_current_user();
		$url = add_query_arg( array(
			'firstName' => $user->first_name,
			'lastName' => $user->last_name,
		));
		wp_redirect( $url );
		exit;
	}
}

add_action( 'template_redirect', 'home_page_first_last_name_redirect' );