diff --git a/ChangeLog b/ChangeLog
index f9ce659246..643ae69700 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Apr 10 15:35:21 2012  Koichi Sasada  <ko1@atdot.net>
+
+	* compile.c (iseq_set_sequence): show a hint if there are duplicated
+	  "when" clauses.  [ruby-core:41502] [ruby-trunk - Feature #5716]
+
 Tue Apr 10 09:57:00 2012  Eric Hodel  <drbrain@segment7.net>
 
 	* string.c (rb_str_split_m):  Documented behavior of split on the empty
diff --git a/compile.c b/compile.c
index db8e48b6c6..b885e0724d 100644
--- a/compile.c
+++ b/compile.c
@@ -1454,8 +1454,9 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *anchor)
 				    rb_hash_aset(map, obj, INT2FIX(lobj->position - (pos+len)));
 				}
 				else {
+				    int n = i/2 + 1;
 				    rb_compile_warning(RSTRING_PTR(iseq->filename), iobj->line_no,
-						       "duplicated when clause is ignored");
+						       "duplicated when clause (#%d) is ignored", n);
 				}
 			    }
 			    hide_obj(map);