feat: add unique constraint to coursestaffrole

edx_exams/apps/core/migrations/0027_coursestaffrole_unique_course_staff_role.py

@@ -0,0 +1,17 @@
+# Generated by Django 4.2.14 on 2024-07-24 14:58
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('core', '0026_studentallowance'),
+    ]
+
+    operations = [
+        migrations.AddConstraint(
+            model_name='coursestaffrole',
+            constraint=models.UniqueConstraint(fields=('user', 'course_id', 'role'), name='unique_course_staff_role'),
+        ),
+    ]

edx_exams/apps/core/models.py

@@ -73,6 +73,11 @@ class Meta:
         db_table = 'exams_coursestaffrole'
         verbose_name = 'course staff role'
 
+        constraints = [
+            models.UniqueConstraint(fields=['user', 'course_id', 'role'],
+                                    name='unique_course_staff_role')
+        ]
+
 
 class ProctoringProvider(TimeStampedModel):
     """