This is for EE3 only -- Creates a message on events for logged in users only, to tell them they have already booked the event. Should go in the event_list_display.php, but should be fine in the registration_page_display.php file as well. REQUIRES WP USER INTEGRATION to be active.

EE Already Booked

<?php
  if ( is_user_logged_in() ) {
		
		$current_user = wp_get_current_user();
		//echo $current_user->ID;
		
		$wpdb->get_results("SELECT id FROM ". EVENTS_MEMBER_REL_TABLE . " WHERE user_id = '" . $current_user->ID . "'");
		
		if ($wpdb->num_rows > 0) {
			$events = $wpdb->get_results("SELECT e.id event_id, e.event_name, e.event_code, e.start_date, e.event_desc, e.display_desc, a.id attendee_id, a.event_time start_time, a.payment_status, a.payment_date, a.amount_pd, u.user_id user_id, a.registration_id, a.lname, a.lname, a.price_option, a.event_time
			FROM " . EVENTS_ATTENDEE_TABLE . " a
			JOIN " . EVENTS_MEMBER_REL_TABLE . " u ON u.attendee_id = a.id
			JOIN " . EVENTS_DETAIL_TABLE . " e ON e.id = u.event_id
			WHERE u.user_id = '" . $current_user->ID . "'");
		}
		
		//var_dump($events);
		
		$event_list_id = array();
		
		foreach ($events as $eve) {
			if ( $eve->user_id == $current_user->ID) {
				$event_list_id[] = $eve->event_id;
			}
		}
		
	}
	//var_dump($event_list_id);
	
		if($event_list_id == NULL) {} else {
	foreach ($event_list_id as $id) {
		if ($id == $event_id) {
			echo '<p style="background-color:red; color:#fff; font-size:20px; text-align:center;">Hey you already booked this!</p>';
			break;
		}
	}
	   }

	
?>