add unique_together on EventMetaProperty (organizer, name)

src/pretix/base/models/event.py

@@ -1841,6 +1841,7 @@ class EventMetaProperty(LoggedModel):
 
     class Meta:
         ordering = ("position", "name",)
+        unique_together = ('organizer', 'name')
 
     @property
     def choice_keys(self):