diff --git a/addons/web/common/http.py b/addons/web/common/http.py
index 6d4bf1536a2..016aff3754e 100644
--- a/addons/web/common/http.py
+++ b/addons/web/common/http.py
@@ -355,7 +355,7 @@ def session_context(request, storage_path, session_cookie='sessionid'):
         # Remove all OpenERPSession instances with no uid, they're generated
         # either by login process or by HTTP requests without an OpenERP
         # session id, and are generally noise
-        removed_sessions = []
+        removed_sessions = set()
         for key, value in request.session.items():
             if (isinstance(value, session.OpenERPSession) 
                 and not value._uid
@@ -363,7 +363,7 @@ def session_context(request, storage_path, session_cookie='sessionid'):
                 and value._creation_time + (60*5) < time.time()  # FIXME do not use a fixed value
             ):
                 _logger.debug('remove session %s', key)
-                removed_sessions.append(key)
+                removed_sessions.add(key)
                 del request.session[key]
 
         with session_lock: