title: Django signals dispatch_uid
body: <p>I have a question regarding the usage of <code>dispatch_uid</code> for signals.</p>
  
  <p>Currently, I am preventing multiple usage of the signal by simply adding <code>if not instance.order_reference</code>. I wonder now if <code>dispatch_uid</code> has the same functionality and I can delete the <em>"<code>if not</code>"</em> clause.</p>
  
  <p><strong>signals.py</strong></p>
  
  <pre class="lang-py prettyprint-override"><code>def reserveditem_create_order_reference(sender, instance, **kwargs):
      if not instance.order_reference:
          instance.order_reference = unique_order_reference_generator()
  </code></pre>
  
  <p><strong>app.py</strong></p>
  
  <pre class="lang-py prettyprint-override"><code>class OrdersConfig(AppConfig):
      name = 'orders'
  
      def ready(self):
  
          #Pre save signal for ReservedItem model
          reserved_model = self.get_model('ReservedItem')
          pre_save.connect(
              reserveditem_create_order_reference,
              sender=reserved_model,
              dispatch_uid="my_unique_identifier"
          )
  </code></pre>