title: Django SlugField not unique
body: <p>I'm stuck from a couple hours and I can't solve this problem. </p>
  
  <p>The following code works well, but if I write a "Title" that already exist I get:</p>
  
  <pre><code>UNIQUE constraint failed: appname_shopaccount.url_shop
  </code></pre>
  
  <p>Model</p>
  
  <pre><code>class ShopAccount(models.Model):
      user = models.ForeignKey(User)
      title = models.CharField(max_length=150)
      url_shop = models.SlugField(max_length=200, unique=True)
      company_name = models.CharField(max_length=200)
  
      def save(self, *args, **kwargs):
          self.url_shop = slugify(self.title)
          super(ShopAccount, self).save(*args, **kwargs)    
  
      def __str__(self):
          return self.title
  </code></pre>
  
  <p>Forms</p>
  
  <pre><code>class SignUpShopForm(ModelForm):
  
      class Meta:
          model = ShopAccount
          fields=['title', 'company_name']        
          exclude= ('user',)
  
          error_messages = {
              'title': {
                  'required': "Enter a Shop name",
              },                                    
              'company_name': {
                  'required': "Enter a Company name",
              }
          }
  </code></pre>
  
  <p>View</p>
  
  <pre><code>def signup_shop(request):    
      if request.POST:
          form = SignUpShopForm(request.POST)
          if form.is_valid():
              account = form.save(commit=False)
              account.user = request.user
              account.save()
              return HttpResponseRedirect('/account/updated/')
          else:
             form = SignUpShopForm(data=request.POST)
             return render_to_response('register_shop.html', { 'form':form }, context_instance=RequestContext(request))
      else:        
          return render_to_response('register_shop.html', context_instance=RequestContext(request)) 
  </code></pre>
  
  <p>How can I solve this problem? </p>