Check if user is already registered for event in The Events Calendar / Event Tickets, in this example the check is included in the orders-link.php template file which you can override by including it in a tribe-events/tickets folder in your theme

orders-link.php

<?php

$view          = Tribe__Tickets__Tickets_View::instance();
$event_id      = get_the_ID();
$user_id       = get_current_user_id();
$user_has_rsvp = $view->has_rsvp_attendees( $event_id, $user_id );