Make the IndexUpdate unique for an Unit

We don't need to update the unit more times, just fetching the latest
state is enough.
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/models/search.py

@@ -23,7 +23,7 @@ from weblate.trans.models.unit import Unit
 
 
 class IndexUpdate(models.Model):
-    unit = models.ForeignKey(Unit)
+    unit = models.ForeignKey(Unit, unique=True)
     source = models.BooleanField(default=True)
 
     class Meta(object):