diff -r e89b3603d3da -r e7af56a0733e hgext/rebase.py
--- a/hgext/rebase.py	Wed Apr 01 13:27:28 2020 -0700
+++ b/hgext/rebase.py	Wed Apr 01 14:34:21 2020 -0700
@@ -631,6 +631,12 @@
                 editor = cmdutil.getcommiteditor(
                     editform=editform, **pycompat.strkwargs(opts)
                 )
+                # We need to set parents again here just in case we're continuing
+                # a rebase started with an old hg version (before 9c9cfecd4600),
+                # because those old versions would have left us with two dirstate
+                # parents, and we don't want to create a merge commit here (unless
+                # we're rebasing a merge commit).
+                self.wctx.setparents(repo[p1].node(), repo[p2].node())
                 newnode = self._concludenode(rev, p1, editor)
             else:
                 # Skip commit if we are collapsing