--- a/mercurial/unionrepo.py	Wed Oct 07 14:26:47 2020 +0530
+++ b/mercurial/unionrepo.py	Sun Oct 18 22:18:02 2020 +0200
@@ -129,6 +129,7 @@
         linkmapper,
         transaction,
         addrevisioncb=None,
+        duplicaterevisioncb=None,
         maybemissingparents=False,
     ):
         raise NotImplementedError