diff --git a/ChangeLog b/ChangeLog
index 1f7ada7176..7d3bea9186 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sun Sep 28 22:05:07 2008  Yusuke Endoh  <mame@tsg.ne.jp>
+
+	* compile.c (iseq_set_sequence): check duplicated when clause.
+	  [ruby-dev:36616]
+
 Sun Sep 28 19:04:03 2008  Tadayoshi Funaba  <tadf@dotrb.org>
 
 	* test/date/*.rb: imported a date test suite ruby 1.9 limited ed.
diff --git a/compile.c b/compile.c
index 4a1fc9c73c..ae93c8e135 100644
--- a/compile.c
+++ b/compile.c
@@ -1349,7 +1349,12 @@ iseq_set_sequence(rb_iseq_t *iseq, LINK_ANCHOR *anchor)
 				    rb_compile_error(RSTRING_PTR(iseq->filename), iobj->line_no,
 						     "unknown label");
 				}
-				rb_hash_aset(map, obj, INT2FIX(lobj->position - (pos+len)));
+				if (!st_lookup(rb_hash_tbl(map), obj, 0)) {
+				    rb_hash_aset(map, obj, INT2FIX(lobj->position - (pos+len)));
+				}
+				else {
+				    rb_warning("duplicated when clause is ignored");
+				}
 			    }
 			    generated_iseq[pos + 1 + j] = map;
 			    iseq_add_mark_object(iseq, map);