title: Django migration with uuid field generates duplicated values
body: <p>I have a <code>uuid</code> field (not a primary key). The generated migration is:</p>
  
  <pre><code>from __future__ import unicode_literals
  
  from django.db import migrations, models
  import uuid
  
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ....
      ]
  
      operations = [
          ...
          migrations.AddField(
              model_name='device',
              name='uuid',
              field=models.UUIDField(default=uuid.uuid4, unique=True),
          ),
          ...
      ]
  </code></pre>
  
  <p>But when doing <code>python manage.py migrate</code> it is crashing with:</p>
  
  <blockquote>
    <p>django.db.utils.IntegrityError: could not create unique index
    "restaurants_device_uuid_key" DETAIL:  Key
    (uuid)=(f3858ded-b8e0-4ac0-8436-8a61b10efc73) is duplicated.</p>
  </blockquote>
  
  <p>Strangely enough, the problem does not seem to occur with primary keys (which are maybe created by the database, and not internally by django?)</p>
  
  <p>How can I add a uuid field, and make sure that migrations work?</p>