diff --git a/swh/loader/mercurial/from_disk.py b/swh/loader/mercurial/from_disk.py
--- a/swh/loader/mercurial/from_disk.py
+++ b/swh/loader/mercurial/from_disk.py
@@ -304,6 +304,15 @@
                 except KeyError:  # the node does not exist anymore
                     pass
 
+            # Mercurial can have more than one head per branch, so we need to exclude
+            # local heads that have already been loaded as revisions but don't
+            # correspond to a SnapshotBranch.
+            # In the future, if the SnapshotBranch model evolves to support multiple
+            # heads per branch (or anything else that fixes this issue) this might
+            # become useless.
+            extids = self.storage.extid_get_from_extid(EXTID_TYPE, repo.heads())
+            known_heads = {extid.extid for extid in extids}
+            existing_heads.extend([repo[head].rev() for head in known_heads])
             # select revisions that are not ancestors of heads
             # and not the heads themselves
             new_revs = repo.revs("not ::(%ld)", existing_heads)
@@ -411,7 +420,7 @@
         from_storage = self.storage.extid_get_from_extid(EXTID_TYPE, ids=[hg_nodeid])
 
         msg = "Expected 1 match from storage for hg node %r, got %d"
-        assert len(from_storage) == 1, msg % (hg_nodeid, len(from_storage))
+        assert len(from_storage) == 1, msg % (hg_nodeid.hex(), len(from_storage))
         return from_storage[0].target.object_id
 
     def get_revision_parents(self, rev_ctx: hgutil.BaseContext) -> Tuple[Sha1Git, ...]:
diff --git a/swh/loader/mercurial/tests/test_from_disk.py b/swh/loader/mercurial/tests/test_from_disk.py
--- a/swh/loader/mercurial/tests/test_from_disk.py
+++ b/swh/loader/mercurial/tests/test_from_disk.py
@@ -11,6 +11,7 @@
 import attr
 import pytest
 
+from swh.loader.mercurial.loader import HgBundle20Loader
 from swh.loader.mercurial.utils import parse_visit_date
 from swh.loader.tests import (
     assert_last_visit_matches,
@@ -408,6 +409,36 @@
     assert get_stats(loader.storage) == {**expected_stats, "origin_visit": 2 + 1}
 
 
+def test_old_loader_new_loader(swh_storage, datadir, tmp_path):
+    archive_name = "example"
+    archive_path = os.path.join(datadir, f"{archive_name}.tgz")
+    repo_url = prepare_repository_from_archive(archive_path, archive_name, tmp_path)
+    repo_path = repo_url.replace("file://", "")
+
+    old_loader = HgBundle20Loader(swh_storage, repo_path)
+    assert old_loader.load() == {"status": "eventful"}
+
+    expected_stats = {
+        "content": 7,
+        "directory": 16,
+        "origin": 1,
+        "origin_visit": 1,
+        "release": 0,
+        "revision": 9,
+        "skipped_content": 0,
+        "snapshot": 1,
+    }
+    assert get_stats(old_loader.storage) == expected_stats
+
+    # Shouldn't pick up anything
+    loader = HgLoaderFromDisk(swh_storage, repo_path)
+    assert loader.load() == {"status": "uneventful"}
+
+    # Shouldn't pick up anything either
+    loader = HgLoaderFromDisk(swh_storage, repo_path)
+    assert loader.load() == {"status": "uneventful"}
+
+
 def test_load_unchanged_repo__dangling_extid(swh_storage, datadir, tmp_path):
     """Checks the loader will load revisions targeted by an ExtID if the
     revisions are missing from the storage"""