Remove unique constraint on NHS number (#414)

# What is the change?

Remove unique constraint on user NHS number

# Why are we making this change?

At p5 level NHS login identities are only "claimed" identities and
multiple users may have the same NHS number (p9 is verified identity).
For pilot we will maintain multiple that multiple NHS numbers may exist
for different users in the system.

We should review post pilot and have a view to how to handle NHS number
duplicates for national launch and what this means for sending data to
other systems or for further care.

Currently the system uses the OIDC sub as a unique identifier so there
are no security concerns around one user seing another users data in the
case where 2 users have the same NHS number

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