diff --git a/src/calibre/db/cache.py b/src/calibre/db/cache.py
index 80b925e770..d3164b5ae1 100644
--- a/src/calibre/db/cache.py
+++ b/src/calibre/db/cache.py
@@ -2350,11 +2350,18 @@ class Cache:
         return self._books_for_field(f.name, int(item_id_or_composite_value))
 
     @read_api
-    def split_if_is_multiple_composite(self, f, v):
+    def split_if_is_multiple_composite(self, f, val):
+        '''
+        If f is a composite column lookup key and the column is is_multiple then
+        split v into unique non-empty values. The comparison is case sensitive.
+        Order is not preserved. Return a list() for compatibility with proxy
+        metadata field getters, for example tags.
+        '''
         fm = self.field_metadata.get(f, None)
         if fm and fm['datatype'] == 'composite' and fm['is_multiple']:
-            return [v.strip() for v in v.split(',') if v.strip()]
-        return v
+            sep = fm['is_multiple'].get('cache_to_list', ',')
+            return (list(set(v.strip() for v in val.split(sep) if v.strip())))
+        return val
 
     @read_api
     def data_for_find_identical_books(self):
diff --git a/src/calibre/gui2/dialogs/quickview.py b/src/calibre/gui2/dialogs/quickview.py
index 2a17db7c2d..3c77b86a12 100644
--- a/src/calibre/gui2/dialogs/quickview.py
+++ b/src/calibre/gui2/dialogs/quickview.py
@@ -542,10 +542,7 @@ class Quickview(QDialog, Ui_Quickview):
         self.books_table.setRowCount(0)
 
         mi = self.db.new_api.get_proxy_metadata(book_id)
-        vals = mi.get(key, None)
-        if self.fm[key]['datatype'] == 'composite' and self.fm[key]['is_multiple']:
-            sep = self.fm[key]['is_multiple'].get('cache_to_list', ',')
-            vals = [v.strip() for v in vals.split(sep) if v.strip()]
+        vals = self.db.new_api.split_if_is_multiple_composite(key, mi.get(key, None))
         try:
             # Check if we are in the GridView and there are no values for the
             # selected column. In this case switch the column to 'authors'
diff --git a/src/calibre/gui2/library/delegates.py b/src/calibre/gui2/library/delegates.py
index 4dcfc988b3..eb9ac48bf1 100644
--- a/src/calibre/gui2/library/delegates.py
+++ b/src/calibre/gui2/library/delegates.py
@@ -773,7 +773,7 @@ class CcTemplateDelegate(QStyledItemDelegate):  # {{{
 
     def __init__(self, parent):
         '''
-        Delegate for custom_column bool data.
+        Delegate for composite custom_columns.
         '''
         QStyledItemDelegate.__init__(self, parent)
         self.disallow_edit = gprefs['edit_metadata_templates_only_F2_on_booklist']