diff --git a/tests/test_web.py b/tests/test_web.py
index c4c524a..17b8d93 100644
--- a/tests/test_web.py
+++ b/tests/test_web.py
@@ -354,6 +354,42 @@ class PortalApplicationTests(unittest.TestCase):
         )
         self.assertEqual("403 Forbidden", replay["status"])
 
+    def test_public_registration_duplicate_inputs_never_take_over_identity(self):
+        verifier = FakeRegistrationVerification()
+        self.app.registration_verification = verifier
+        self.app.provisioning = FakeProvisioning()
+        self.app.registration_clients = frozenset({"coulomb-social"})
+        self.app.registration_tenants = frozenset({"tenant:coulomb"})
+        self.app.registration_oidc_issuer = "https://kc.example"
+        self.app.registration_password_setup_origins = frozenset({"https://kc.example"})
+
+        def register(username, email):
+            invoke(self.app, "/api/v1/public/registrations", method="POST", body={
+                "username": username, "email": email,
+                "client_id": "coulomb-social", "tenant": "tenant:coulomb",
+            })
+            registration_id = verifier.requested.registration_id
+            verifier.registration_id = registration_id
+            result, _ = invoke(
+                self.app, "/api/v1/public/registrations/verify", method="POST",
+                body={"handle": "x" * 32},
+            )
+            return result, self.app.service.store.registration_session(registration_id)
+
+        first, first_session = register("first.person", "shared@example.test")
+        second, second_session = register("second.person", "shared@example.test")
+        collision, collision_session = register("first.person", "other@example.test")
+
+        self.assertEqual("303 See Other", first["status"])
+        self.assertEqual("303 See Other", second["status"])
+        self.assertNotEqual(first_session.user_id, second_session.user_id)
+        self.assertEqual("409 Conflict", collision["status"])
+        linked = self.app.service.store.find_identity(
+            "https://kc.example", "first.person"
+        )
+        self.assertEqual(first_session.user_id, linked.user_id)
+        self.assertNotEqual(collision_session.user_id, linked.user_id)
+
     def test_public_registration_rejects_unregistered_client(self):
         self.app.registration_verification = FakeRegistrationVerification()
         self.app.registration_clients = frozenset({"coulomb-social"})
diff --git a/workplans/USER-WP-0022-public-registration-and-jit-application-profiles.md b/workplans/USER-WP-0022-public-registration-and-jit-application-profiles.md
index 4fe0b3f..f09ea0d 100644
--- a/workplans/USER-WP-0022-public-registration-and-jit-application-profiles.md
+++ b/workplans/USER-WP-0022-public-registration-and-jit-application-profiles.md
@@ -197,3 +197,10 @@ verification expiry/replay redaction, peer rate limiting, identity-link
 collision, cross-tenant denial, lifecycle deletion, audit/outbox redaction,
 and request correlation. Deployed consumer conformance remains gated on public
 runtime credentials and activation.
+
+Duplicate-input conformance now also proves that a verified mailbox reused
+with a different username creates a distinct local user instead of implicitly
+linking to an existing account. Reusing an existing provider username fails at
+the identity-link uniqueness boundary, leaves the original link unchanged, and
+never transfers that identity to the later registration. The full suite passes
+131 tests with three environment-dependent skips.