--- a/mercurial/copies.py	Mon Oct 05 01:49:04 2020 +0200
+++ b/mercurial/copies.py	Fri Nov 20 10:49:32 2020 +0100
@@ -366,6 +366,9 @@
             othercopies = all_copies.get(c)
             if othercopies is None:
                 all_copies[c] = newcopies
+            elif newcopies is othercopies:
+                # nothing to merge:
+                pass
             else:
                 # we are the second parent to work on c, we need to merge our
                 # work with the other.