diff -r f0027a3dd7cb -r 218feb1a7e00 mercurial/merge.py
--- a/mercurial/merge.py	Wed Feb 19 17:30:04 2020 +0100
+++ b/mercurial/merge.py	Mon Feb 17 20:30:03 2020 -0500
@@ -2654,10 +2654,15 @@
     # to copy commits), and 2) informs update that the incoming changes are
     # newer than the destination so it doesn't prompt about "remote changed foo
     # which local deleted".
+    # We also pass mergeancestor=True when base is the same revision as p1. 2)
+    # doesn't matter as there can't possibly be conflicts, but 1) is necessary.
     wctx = wctx or repo[None]
     pctx = wctx.p1()
     base = base or ctx.p1()
-    mergeancestor = repo.changelog.isancestor(pctx.node(), ctx.node())
+    mergeancestor = (
+        repo.changelog.isancestor(pctx.node(), ctx.node())
+        or pctx.rev() == base.rev()
+    )
 
     stats = update(
         repo,