diff -r 1e0516ee4cb9 -r b20d04641c0f hgext3rd/evolve/headchecking.py
--- a/hgext3rd/evolve/headchecking.py	Mon Apr 06 05:08:23 2020 +0200
+++ b/hgext3rd/evolve/headchecking.py	Wed Mar 11 23:56:11 2020 +0100
@@ -4,10 +4,15 @@
 
 from mercurial import (
     discovery,
+    error,
     extensions,
     phases,
+    scmutil,
+    util,
 )
 
+from mercurial.i18n import _
+
 
 from . import (
     compat,
@@ -20,6 +25,7 @@
 @eh.uisetup
 def uisetup(ui):
     extensions.wrapfunction(discovery, '_postprocessobsolete', _postprocessobsolete)
+    extensions.wrapfunction(scmutil, 'enforcesinglehead', enforcesinglehead)
 
 def branchinfo(pushop, repo, node):
     return repo[node].branch()
@@ -111,3 +117,71 @@
             discarded.add(nh)
     newhs |= unknownheads
     return newhs, discarded
+
+def _get_branch_name(ctx):
+    # make it easy for extension with the branch logic there
+    branch = ctx.branch()
+    if util.safehasattr(ctx, 'topic'):
+        topic = ctx.topic()
+        if topic:
+            branch = "%s:%s" % (branch, topic)
+    return branch
+
+def _filter_obsolete_heads(repo, heads):
+    """filter heads to return non-obsolete ones
+
+    Given a list of heads (on the same named branch) return a new list of heads
+    where the obsolete part have been skimmed out.
+    """
+    new_heads = []
+    old_heads = heads[:]
+    while old_heads:
+        rh = old_heads.pop()
+        ctx = repo[rh]
+        current_name = _get_branch_name(ctx)
+        # run this check early to skip the evaluation of the whole branch
+        if not ctx.obsolete():
+            new_heads.append(rh)
+            continue
+
+        # Get all revs/nodes on the branch exclusive to this head
+        # (already filtered heads are "ignored"))
+        sections_revs = repo.revs(
+            b'only(%d, (%ld+%ld))', rh, old_heads, new_heads,
+        )
+        keep_revs = []
+        for r in sections_revs:
+            ctx = repo[r]
+            if ctx.obsolete():
+                continue
+            if _get_branch_name(ctx) != current_name:
+                continue
+            keep_revs.append(r)
+        for h in repo.revs(b'heads(%ld and (::%ld))', sections_revs, keep_revs):
+            new_heads.append(h)
+    new_heads.sort()
+    return new_heads
+
+def enforcesinglehead(orig, repo, tr, desc, accountclosed=False):
+    """check that no named branch has multiple heads"""
+    nodesummaries = scmutil.nodesummaries
+    if desc in (b'strip', b'repair'):
+        # skip the logic during strip
+        return
+    visible = repo.filtered(b'visible')
+    # possible improvement: we could restrict the check to affected branch
+    bm = visible.branchmap()
+    cl = repo.changelog
+    to_rev = cl.rev
+    to_node = cl.node
+    for name in bm:
+        all_heads = bm.branchheads(name, closed=accountclosed)
+        all_heads = [to_rev(n) for n in all_heads]
+        heads = _filter_obsolete_heads(repo, all_heads)
+        heads = [to_node(r) for r in heads]
+        if len(heads) > 1:
+            msg = _(b'rejecting multiple heads on branch "%s"')
+            msg %= name
+            hint = _(b'%d heads: %s')
+            hint %= (len(heads), nodesummaries(repo, heads))
+            raise error.Abort(msg, hint=hint)