Fix typo in comment

flask/helpers.py

@@ -383,7 +383,7 @@ def flash(message, category='message'):
     #     session.setdefault('_flashes', []).append((category, message))
     #
     # This assumed that changes made to mutable structures in the session are
-    # are always in sync with the session object, which is not true for session
+    # always in sync with the session object, which is not true for session
     # implementations that use external storage for keeping their keys/values.
     flashes = session.get('_flashes', [])
     flashes.append((category, message))