diff --git a/zerver/lib/push_notifications.py b/zerver/lib/push_notifications.py
index 4b8dbb21fa..0ed00d59dc 100644
--- a/zerver/lib/push_notifications.py
+++ b/zerver/lib/push_notifications.py
@@ -547,6 +547,9 @@ def add_push_device_token(
         post_data = {
             "server_uuid": settings.ZULIP_ORG_ID,
             "user_uuid": str(user_profile.uuid),
+            # user_id is sent so that the bouncer can delete any pre-existing registrations
+            # for this user+device to avoid duplication upon adding the uuid registration.
+            "user_id": str(user_profile.id),
             "token": token_str,
             "token_kind": kind,
         }
diff --git a/zerver/tests/test_push_notifications.py b/zerver/tests/test_push_notifications.py
index a7e85ea8c4..ccdb8b5e9c 100644
--- a/zerver/tests/test_push_notifications.py
+++ b/zerver/tests/test_push_notifications.py
@@ -192,12 +192,6 @@ class PushBouncerNotificationTest(BouncerTestCase):
             self.server_uuid, endpoint, {"token": token, "token_kind": token_kind}
         )
         self.assert_json_error(result, "Missing user_id or user_uuid")
-        result = self.uuid_post(
-            self.server_uuid,
-            endpoint,
-            {"user_id": user_id, "user_uuid": "xxx", "token": token, "token_kind": token_kind},
-        )
-        self.assert_json_error(result, "Specify only one of user_id or user_uuid")
         result = self.uuid_post(
             self.server_uuid, endpoint, {"user_id": user_id, "token": token, "token_kind": 17}
         )
@@ -280,6 +274,40 @@ class PushBouncerNotificationTest(BouncerTestCase):
             401,
         )
 
+    def test_register_device_deduplication(self) -> None:
+        hamlet = self.example_user("hamlet")
+        token = "111222"
+        user_id = hamlet.id
+        user_uuid = str(hamlet.uuid)
+        token_kind = PushDeviceToken.GCM
+
+        endpoint = "/api/v1/remotes/push/register"
+
+        # First we create a legacy user_id registration.
+        result = self.uuid_post(
+            self.server_uuid,
+            endpoint,
+            {"user_id": user_id, "token_kind": token_kind, "token": token},
+        )
+        self.assert_json_success(result)
+
+        registrations = list(RemotePushDeviceToken.objects.filter(token=token))
+        self.assert_length(registrations, 1)
+        self.assertEqual(registrations[0].user_id, user_id)
+        self.assertEqual(registrations[0].user_uuid, None)
+
+        # Register same user+device with uuid now. The old registration should be deleted
+        # to avoid duplication.
+        result = self.uuid_post(
+            self.server_uuid,
+            endpoint,
+            {"user_id": user_id, "user_uuid": user_uuid, "token_kind": token_kind, "token": token},
+        )
+        registrations = list(RemotePushDeviceToken.objects.filter(token=token))
+        self.assert_length(registrations, 1)
+        self.assertEqual(registrations[0].user_id, None)
+        self.assertEqual(str(registrations[0].user_uuid), user_uuid)
+
     def test_remote_push_user_endpoints(self) -> None:
         endpoints = [
             ("/api/v1/remotes/push/register", "register"),
diff --git a/zilencer/views.py b/zilencer/views.py
index dbe9607ecf..50b1ea3c44 100644
--- a/zilencer/views.py
+++ b/zilencer/views.py
@@ -157,23 +157,26 @@ def register_remote_push_device(
     if user_id is None and user_uuid is None:
         raise JsonableError(_("Missing user_id or user_uuid"))
     if user_id is not None and user_uuid is not None:
-        # We don't want "hybrid" registrations with both.
-        # Our RemotePushDeviceToken should be either in the new uuid format
-        # or the legacy id one.
-        raise JsonableError(_("Specify only one of user_id or user_uuid"))
-
+        kwargs: Dict[str, object] = {"user_uuid": user_uuid, "user_id": None}
+        # Delete pre-existing user_id registration for this user+device to avoid
+        # duplication. Further down, uuid registration will be created.
+        RemotePushDeviceToken.objects.filter(
+            server=server, token=token, kind=token_kind, user_id=user_id
+        ).delete()
+    else:
+        # One of these is None, so these will kwargs will leads to a proper registration
+        # of either user_id or user_uuid type
+        kwargs = {"user_id": user_id, "user_uuid": user_uuid}
     try:
         with transaction.atomic():
             RemotePushDeviceToken.objects.create(
-                # Exactly one of these two user identity fields will be None.
-                user_id=user_id,
-                user_uuid=user_uuid,
                 server=server,
                 kind=token_kind,
                 token=token,
                 ios_app_id=ios_app_id,
                 # last_updated is to be renamed to date_created.
                 last_updated=timezone.now(),
+                **kwargs,
             )
     except IntegrityError:
         pass