1 files changed, 11 insertions, 1 deletions
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index 6ffb8c0da6..f287ee247b 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -47,7 +47,8 @@ class RegistrationHandler(BaseHandler):
         self._next_generated_user_id = None
 
     @defer.inlineCallbacks
-    def check_username(self, localpart, guest_access_token=None):
+    def check_username(self, localpart, guest_access_token=None,
+                       assigned_user_id=None):
         yield run_on_reactor()
 
         if urllib.quote(localpart.encode('utf-8')) != localpart:
@@ -60,6 +61,15 @@ class RegistrationHandler(BaseHandler):
         user = UserID(localpart, self.hs.hostname)
         user_id = user.to_string()
 
+        if assigned_user_id:
+            if user_id == assigned_user_id:
+                return
+            else:
+                raise SynapseError(
+                    400,
+                    "A different user ID has already been registered for this session",
+                )
+
         yield self.check_user_id_not_appservice_exclusive(user_id)
 
         users = yield self.store.get_users_by_id_case_insensitive(user_id)