title: Django Check for duplicates with 2 model fields
body: <p>I have a model that looks like this:</p>
  
  <pre><code>class UssdCode(models.Model):
      title = models.CharField(max_length=100)
      code = models.CharField(max_length=100)
      product = models.CharField(max_length=100)
  </code></pre>
  
  <p>How can I get the admin to alert me and reject my entry when I try to add a new object that has the same 'code' and 'product' as an object already in the database.</p>