Remove unique constraint on NHS number

At p5 level NHS login identities are only "claimed" identities. For
pilot we will mainatin multiple that multiple NHS numbers may exist.

We should review post pilot and have a view to how to handle NHS number
duplicates for national launch

Author: Themitchell

lung_cancer_screening/questions/migrations/0009_alter_sexatbirthresponse_value_alter_user_nhs_number.py

@@ -0,0 +1,23 @@
+# Generated by Django 6.0.3 on 2026-03-26 15:10
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('questions', '0008_alter_genderresponse_value'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='sexatbirthresponse',
+            name='value',
+            field=models.CharField(choices=[('F', 'Female'), ('M', 'Male')], max_length=1),
+        ),
+        migrations.AlterField(
+            model_name='user',
+            name='nhs_number',
+            field=models.CharField(max_length=10),
+        ),
+    ]

lung_cancer_screening/questions/models/user.py

@@ -20,7 +20,7 @@ def create_user(self, sub, **extra_fields):
 
 class User(AbstractBaseUser):
     sub = models.CharField(max_length=255, unique=True)
-    nhs_number = models.CharField(max_length=10, unique=True)
+    nhs_number = models.CharField(max_length=10)
     given_name = models.CharField(max_length=255)
     family_name = models.CharField(max_length=255)
     email = models.EmailField()

lung_cancer_screening/questions/tests/unit/models/test_user.py

@@ -86,16 +86,6 @@ def test_is_invalid_without_nhs_number(self):
         )
 
 
-    def test_raises_a_validation_error_if_nhs_number_is_duplicate(self):
-        with self.assertRaises(ValidationError) as context:
-            UserFactory(nhs_number=self.user.nhs_number)
-
-        self.assertIn(
-            "User with this Nhs number already exists.",
-            context.exception.messages
-        )
-
-
     def test_nhs_number_has_a_max_length_of_10(self):
         self.user.nhs_number = "1"*11