screning number is unique at database level

smash/web/models.py

@@ -116,6 +116,7 @@ class Subject(models.Model):
     )
     screening_number = models.CharField(max_length=50,
 	    blank=True,
+	    unique=True,
         verbose_name='Screening number'
     )
     nd_number = models.CharField(max_length=6,