diff --git a/src/calibre/library/field_metadata.py b/src/calibre/library/field_metadata.py
index 474f70d500..a7d05d396a 100644
--- a/src/calibre/library/field_metadata.py
+++ b/src/calibre/library/field_metadata.py
@@ -7,7 +7,6 @@ import copy
 
 from calibre.utils.ordered_dict import OrderedDict
 from calibre.utils.config import tweaks
-from calibre.utils.icu import lower
 
 class TagsIcons(dict):
     '''
@@ -478,8 +477,6 @@ class FieldMetadata(dict):
                 del self._tb_cats[key]
                 if key in self._search_term_map:
                     del self._search_term_map[key]
-                if key in self._search_term_map:
-                    del self._search_term_map[key]
 
     def cc_series_index_column_for(self, key):
         return self._tb_cats[key]['rec_index'] + 1