diff --git a/satella/coding/concurrent/callablegroup.py b/satella/coding/concurrent/callablegroup.py
index d08ae8ad8a41aede23adf6508f190105f27ec8a5..4f468223296dd13362c50f82417a12e639261565 100644
--- a/satella/coding/concurrent/callablegroup.py
+++ b/satella/coding/concurrent/callablegroup.py
@@ -111,6 +111,8 @@ class CallableGroup(tp.Generic[T]):
         """
         Add a callable.
 
+        .. note:: Same callable can't be added twice. It will silently fail.
+
         Can be a :class:`~satella.coding.concurrent.CancellableCallback`, in that case
         method :meth:`~satella.coding.concurrent.CallableGroup.remove_cancelled` might
         be useful.