title: Issue with Django admin registering an inline user profile admin
body: <p>I'm currently working on a django project. I'm attempting to add a UserProfile model inline to my User model. In my models.py I have:</p>
  
  <pre><code>class UserProfile(models.Model):
      '''
      Extension to the User model in django admin.
      '''
      user = models.ForeignKey(User)
      site_role = models.CharField(max_length=128, choices=SITE_ROLE)
      signature = models.CharField(max_length=128)
      position_title = models.CharField(max_length=128)
      on_duty = models.BooleanField(default=False)
      on_duty_order = models.IntegerField()
  </code></pre>
  
  <p>In my admin.py I have:</p>
  
  <pre><code>class UserProfileInline(admin.StackedInline):
      model = UserProfile
  
  class UserAdmin(admin.ModelAdmin):
      inlines = [UserProfileInline]
  
  
  admin.site.unregister(User)
  admin.site.register(User, UserAdmin)
  </code></pre>
  
  <p>When I run the development server (yes, I have restarted it) I get the following exception:</p>
  
  <pre><code>NotRegistered at /admin
  The model User is not registered
  </code></pre>
  
  <p>This exception is coming from the <code>admin.site.unregister(User)</code> line.</p>
  
  <p>However, when I comment out that line, I get the following exception:</p>
  
  <pre><code>AlreadyRegistered at /admin
  The model User is already registered
  </code></pre>
  
  <p>Something about my django setup seems to be a little bi-polar. I've spent an hour or so researching this problem and the code I have seems to work great for others. Does anyone have any insight into why this might be happening?</p>
  
  <p>Thanks,
     Travis</p>