trigger CheckDupeEmails on Contact (before insert, before update) {
    Map<String, Contact> contactMap = new Map<String, Contact>();
    for (Contact Contact:System.Trigger.new) {
        if ((Contact.Email != null) && (System.Trigger.isInsert || Contact.Email != System.Trigger.oldMap.get(Contact.Id).Email)) {
            if (contactMap.containsKey(Contact.Email)) {
                Contact.Email.addError('Another new Contact has the same email address.');
            } else {
                contactMap.put(Contact.Email, Contact);
            }
        }
    }
    
    for (Contact contact : [SELECT Email FROM Contact WHERE Email IN :contactMap.KeySet()]) {
        Contact newContact = contactMap.get(Contact.Email);
        newContact.Email.addError('A Contact with this email address already exists');
    }
}