--- a/mercurial/cmdutil.py	Thu Nov 15 22:28:38 2018 -0500
+++ b/mercurial/cmdutil.py	Fri Nov 09 13:57:13 2018 +0800
@@ -732,11 +732,10 @@
         rewriteutil.precheck(repo, revs, 'change branch of')
 
         root = repo[roots.first()]
-        if not root.p1().branch() == label and label in repo.branchmap():
+        rpb = {parent.branch() for parent in root.parents()}
+        if label not in rpb and label in repo.branchmap():
             raise error.Abort(_("a branch of the same name already exists"))
 
-        if repo.revs('merge() and %ld', revs):
-            raise error.Abort(_("cannot change branch of a merge commit"))
         if repo.revs('obsolete() and %ld', revs):
             raise error.Abort(_("cannot change branch of a obsolete changeset"))