<?php

add_filter( 'em_event_save_events_recurrence_slug', 'em_repeating_event_slug', 100, 6 );
function em_repeating_event_slug( $event_url, $event_slug, $event_slug_date, $day, $EM_Event, $Recurrence_Set ){
 $event_url = $EM_Event->event_slug . '-' . $event_slug_date;
 return $event_url;
}