diff --git a/acknowledgements.txt b/acknowledgements.txt
index 0d70914839af..d2f4ee91c4de 100644
--- a/acknowledgements.txt
+++ b/acknowledgements.txt
@@ -116,6 +116,8 @@
 2011 Jul 09
      Stefan Birkner: Fixed rules bugs (github#257, gihub#258, github#260).
 
+2011 Jul 09
+     Stefan Birkner: Fixed rules bugs (github#257, gihub#258, github#260).
 2011 Jul 22
      Andreas Köhler, Stefan Birkner: Fixed wrong documentation of Parameterized (github#89).