diff --git a/lung_cancer_screening/questions/migrations/0009_alter_sexatbirthresponse_value_alter_user_nhs_number.py b/lung_cancer_screening/questions/migrations/0009_alter_sexatbirthresponse_value_alter_user_nhs_number.py
new file mode 100644
index 00000000..9a4fd51c
--- /dev/null
+++ b/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),
+        ),
+    ]
diff --git a/lung_cancer_screening/questions/models/user.py b/lung_cancer_screening/questions/models/user.py
index 8bb5fd0e..ff3ed217 100644
--- a/lung_cancer_screening/questions/models/user.py
+++ b/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()
diff --git a/lung_cancer_screening/questions/tests/unit/models/test_user.py b/lung_cancer_screening/questions/tests/unit/models/test_user.py
index 00d797d5..1f02ffbb 100644
--- a/lung_cancer_screening/questions/tests/unit/models/test_user.py
+++ b/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