trigger ReservationTrigger on Reservations__c (after insert) {
    // After Insert, if the Status is confirmed update the Contact
    Set<Id> contactIds = new Set<Id>();
    for(Reservations__c oneReservation:trigger.new()){
        if(oneReservation.ReservationStatus__c == 'Confirmed'){
            contactIds.add(oneReservation.Id);
        }
    }
    // Get a list of matching contacts  
    List<Contact> existingContacts = [Select Id, Repeater__c,
                                        (Select id from Reservations__r)  
                                        From Contact 
                                        Where Id in:contactIds
                                        And Repeater__c = 'False'];

    // Itterate through the contacts
    List<Contact >contactsToUpdate = new List<Contact>();
    if(existingContacts.size()>0){
        for(Contact oneContact: existingContacts){
            // Check if the Contact has two Reservations, we only want to update on the secon
            List<Reservations__c> existingReservations = oneContact.getSObjects('Reservations__r');
            if(existingReservations.size() == 2){
                oneContact.Repeater__c = True;
                contactsToUpdate.add(oneContact);
            }
        }
        // Update the contacts
        Update contactsToUpdate;
    }
}