diff --git a/mercurial/merge.py b/mercurial/merge.py
--- a/mercurial/merge.py
+++ b/mercurial/merge.py
@@ -139,6 +139,8 @@ def findcopies(repo, m1, m2, ma, limit):
         ''' check if an apparent pair actually matches '''
         c2 = ctx(f2, man[f2])
         ca = c.ancestor(c2)
+        if c == ca or c2 == ca:
+            return
         if ca and ca.path() == c.path() or ca.path() == c2.path():
             copy[c.path()] = f2
             copy[f2] = c.path()