Index: django/contrib/admin/media/js/SelectBox.js
===================================================================
--- django/contrib/admin/media/js/SelectBox.js	(revision 4144)
+++ django/contrib/admin/media/js/SelectBox.js	(working copy)
@@ -80,8 +80,10 @@
         var to_box = document.getElementById(to);
         var option;
         for (var i = 0; (option = from_box.options[i]); i++) {
-            SelectBox.add_to_cache(to, { value: option.value, text: option.text, displayed: 1 });
-            SelectBox.delete_from_cache(from, option.value);
+            if (SelectBox.cache_contains(from, option.value)) {
+                SelectBox.add_to_cache(to, { value: option.value, text: option.text, displayed: 1 });
+                SelectBox.delete_from_cache(from, option.value);
+            }
         }
         SelectBox.redisplay(from);
         SelectBox.redisplay(to);