diff -r bc775b8cc020 -r 66ecde8a704d mercurial/scmutil.py
--- a/mercurial/scmutil.py	Thu Nov 16 03:52:38 2017 +0100
+++ b/mercurial/scmutil.py	Thu Nov 16 03:52:42 2017 +0100
@@ -1286,6 +1286,21 @@
     first = ' '.join(short(h) for h in nodes[:maxnumnodes])
     return _("%s and %s others") % (first, len(nodes) - maxnumnodes)
 
+def enforcesinglehead(repo, tr, desc):
+    """check that no named branch has multiple heads"""
+    if desc in ('strip', 'repair'):
+        # skip the logic during strip
+        return
+    visible = repo.filtered('visible')
+    # possible improvement: we could restrict the check to affected branch
+    for name, heads in visible.branchmap().iteritems():
+        if len(heads) > 1:
+            msg = _('rejecting multiple heads on branch "%s"')
+            msg %= name
+            hint = _('%d heads: %s')
+            hint %= (len(heads), nodesummaries(repo, heads))
+            raise error.Abort(msg, hint=hint)
+
 def wrapconvertsink(sink):
     """Allow extensions to wrap the sink returned by convcmd.convertsink()
     before it is used, whether or not the convert extension was formally loaded.