diff --git a/src/calibre/library/database2.py b/src/calibre/library/database2.py
index 72ff9cd08d..47bde01a81 100644
--- a/src/calibre/library/database2.py
+++ b/src/calibre/library/database2.py
@@ -3289,7 +3289,8 @@ class LibraryDatabase2(LibraryDatabase, SchemaUpgrade, CustomColumns):
             paths    = list(duplicate[0] for duplicate in duplicates)
             formats  = list(duplicate[1] for duplicate in duplicates)
             metadata = list(duplicate[2] for duplicate in duplicates)
-            return (paths, formats, metadata), len(ids)
+            return (paths, formats, metadata), (ids if return_ids else
+                    len(ids))
         return None, (ids if return_ids else len(ids))
 
     def import_book(self, mi, formats, notify=True, import_hooks=True,