Make entries in additional registration fields unique.

ticdesk_events/models.py

@@ -22,6 +22,7 @@ class Voucher(models.Model):
 class RegistrationField(models.Model):
     class Meta:
         app_label = 'ticdesk_events'
+        unique_together = ('project_cn', 'person_uid', 'title')
 
     project_cn = models.CharField(max_length=50)
     person_uid = models.CharField(max_length=20)