Ensure the units are unique in the database

This will make the errors appear when they happen rather than later.

Issue #1080
Signed-off-by: Michal Čihař <michal@cihar.com>

weblate/trans/migrations/0062_auto_20160419_1429.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.9.5 on 2016-04-19 14:29
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('trans', '0061_auto_20160404_1841'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='unit',
+            unique_together=set([('translation', 'checksum')]),
+        ),
+    ]

weblate/trans/models/unit.py

@@ -357,6 +357,7 @@ class Unit(models.Model, LoggerMixin):
         )
         ordering = ['priority', 'position']
         app_label = 'trans'
+        unique_together = ('translation', 'checksum')
 
     def __init__(self, *args, **kwargs):
         """