Added a constrain for clientfile

cvat/apps/engine/migrations/0026_auto_20190206_0025.py

@@ -0,0 +1,23 @@
+# Generated by Django 2.1.5 on 2019-02-05 21:25
+
+import cvat.apps.engine.models
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('engine', '0025_auto_20190205_2256'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='clientfile',
+            name='file',
+            field=models.FileField(storage=cvat.apps.engine.models.MyFileSystemStorage(), upload_to=cvat.apps.engine.models.upload_path_handler),
+        ),
+        migrations.AlterUniqueTogether(
+            name='clientfile',
+            unique_together={('task', 'file')},
+        ),
+    ]

cvat/apps/engine/models.py

@@ -106,6 +106,7 @@ class ClientFile(models.Model):
 
     class Meta:
         default_permissions = ()
+        unique_together = ("task", "file")
 
 # For server files on the mounted share
 class ServerFile(models.Model):