title: An issue with admin.autodiscover()
body: <p>I am having a problem with <code>admin.autodiscover()</code>. I just moved my site from the system I was building it on to the server it will stay on for production use. The system that was being used to build it is Windows 7 however the production server is Debian Squeeze 64bit.</p>
  
  <p>The problem is that I am getting the error <code>The model UserProfile is already registered</code>. I am not having this problem with the Windows 7 computer, only once it moves to Debian it does.</p>
  
  <p>admin.py</p>
  
  <pre><code>class UserProfileadmin(admin.ModelAdmin):
      list_display = ['user', 'country', 'totalflights', 'totalhours', 'hub']
      search_fields = ['user']
      list_filter = ['hub']
      def formfield_for_foreignkey(self, db_field, request, **kwargs):
           if db_field.name == 'hub':
               kwargs['queryset'] = Airports.objects.filter(hub=True)
  
      return super(UserProfileadmin, self).formfield_for_foreignkey(db_field, request, **kwargs)
  
  admin.site.register(UserProfile,UserProfileadmin)
  class PilotRankadmin(admin.ModelAdmin):
      list_display = ['ranktitle', 'minimumhours', 'pay']
      search_fields = ['ranktitle']
  
  admin.site.register(PilotRank, PilotRankadmin)
  
  class PIREPadmin(admin.ModelAdmin):
      list_display = ['user', 'flightnumber']
      search_fields = ['ranktitle']
  
  admin.site.register(PIREP, PIREPadmin)
  </code></pre>
  
  <p>Any ideas? I have never had this before when working on a Windows system then moving to a Linux system for production.</p>