diff --git a/boards/views/__init__.py b/boards/views/__init__.py
--- a/boards/views/__init__.py
+++ b/boards/views/__init__.py
@@ -513,6 +513,10 @@ def _get_user(request):
         md5.update(session.session_key)
         new_id = md5.hexdigest()
 
+        while User.objects.filter(user_id=new_id).exists():
+            md5.update(str(timezone.now()))
+            new_id = md5.hexdigest()
+
         time_now = timezone.now()
         user = User.objects.create(user_id=new_id, rank=RANK_USER,
                                    registration_time=time_now)