title: Django - using multiple foreign key to the same model
body: <p>I'm trying to use the same foreign key for two fields in the same model and am getting an error.</p>
  
  <p>Im trying to have a primary and secondary on call user, but am not sure how to format the relationship after receiving the errors below </p>
  
  <pre><code>class ManualRotas(models.Model):
      rota_name = models.CharField(max_length=200,choices=settings.ONCALL_ROTAS)
      primary_user = models.ForeignKey(User, unique=True, verbose_name="Primary OnCall Engineer")
      p_start_time = models.DateTimeField(verbose_name="Start Time")
      p_end_time = models.DateTimeField(verbose_name="End Time")
      secondary_user = models.ForeignKey(User, verbose_name="Backup OnCall Engineer", unique=True,blank=True,null=True)
      s_start_time = models.DateTimeField(blank=True,null=True, verbose_name="Start Time")
      s_end_time = models.DateTimeField(blank=True,null=True,verbose_name="Start Time")
  
  
  ERRORS:
  oncall.ManualRotas.primary_user: (fields.E304) Reverse accessor for 'ManualRotas.primary_user' clashes with reverse accessor for 'ManualRotas.secondary_user'.
          HINT: Add or change a related_name argument to the definition for 'ManualRotas.primary_user' or 'ManualRotas.secondary_user'.
  oncall.ManualRotas.secondary_user: (fields.E304) Reverse accessor for 'ManualRotas.secondary_user' clashes with reverse accessor for 'ManualRotas.primary_user'.
          HINT: Add or change a related_name argument to the definition for 'ManualRotas.secondary_user' or 'ManualRotas.primary_user'.
  
  WARNINGS:
  oncall.ManualRotas.primary_user: (fields.W342) Setting unique=True on a ForeignKey has the same effect as using a OneToOneField.
          HINT: ForeignKey(unique=True) is usually better served by a OneToOneField.
  oncall.ManualRotas.secondary_user: (fields.W342) Setting unique=True on a ForeignKey has the same effect as using a OneToOneField.
          HINT: ForeignKey(unique=True) is usually better served by a OneToOneField.
  
  System check identified 4 issues (0 silenced).
  </code></pre>