diff --git a/src/tests/test_sync_race_conditions.py b/src/tests/test_sync_race_conditions.py
new file mode 100644
index 0000000..0786659
--- /dev/null
+++ b/src/tests/test_sync_race_conditions.py
@@ -0,0 +1,98 @@
+import asyncio
+import threading
+from pathlib import Path
+
+
+from helpers import create_vault, dummy_nostr_client
+
+from seedpass.core.entry_management import EntryManager
+from seedpass.core.backup import BackupManager
+from seedpass.core.config_manager import ConfigManager
+from seedpass.core.manager import PasswordManager, EncryptionMode
+
+
+def _init_pm(dir_path: Path, client) -> PasswordManager:
+    vault, enc_mgr = create_vault(dir_path)
+    cfg_mgr = ConfigManager(vault, dir_path)
+    backup_mgr = BackupManager(dir_path, cfg_mgr)
+    entry_mgr = EntryManager(vault, backup_mgr)
+
+    pm = PasswordManager.__new__(PasswordManager)
+    pm.encryption_mode = EncryptionMode.SEED_ONLY
+    pm.encryption_manager = enc_mgr
+    pm.vault = vault
+    pm.entry_manager = entry_mgr
+    pm.backup_manager = backup_mgr
+    pm.config_manager = cfg_mgr
+    pm.nostr_client = client
+    pm.fingerprint_dir = dir_path
+    pm.is_dirty = False
+    pm.state_manager = None
+    return pm
+
+
+def test_sync_race_conditions(monkeypatch, tmp_path):
+    client_a, relay = dummy_nostr_client.__wrapped__(tmp_path / "c1", monkeypatch)
+
+    from cryptography.fernet import Fernet
+    from nostr.client import NostrClient
+    from seedpass.core.encryption import EncryptionManager
+    from helpers import TEST_SEED
+
+    enc_mgr = EncryptionManager(Fernet.generate_key(), tmp_path / "c2")
+
+    class DummyKeys:
+        def private_key_hex(self):
+            return "1" * 64
+
+        def public_key_hex(self):
+            return "2" * 64
+
+    class DummyKeyManager:
+        def __init__(self, *a, **k):
+            self.keys = DummyKeys()
+
+    monkeypatch.setattr("nostr.client.KeyManager", DummyKeyManager)
+    monkeypatch.setattr(enc_mgr, "decrypt_parent_seed", lambda: TEST_SEED)
+    client_b = NostrClient(enc_mgr, "fp")
+
+    dir_a = tmp_path / "A"
+    dir_b = tmp_path / "B"
+    dir_a.mkdir()
+    dir_b.mkdir()
+
+    pm_a = _init_pm(dir_a, client_a)
+    pm_b = _init_pm(dir_b, client_b)
+
+    pm_a.entry_manager.add_entry("init", 12)
+    pm_a.sync_vault()
+    manifest_id = relay.manifests[-1].tags[0]
+    assert pm_b.attempt_initial_sync() is True
+
+    pm_b.entry_manager.get_next_index = lambda: 2
+
+    def publish(pm: PasswordManager, client, label: str) -> None:
+        pm.entry_manager.add_entry(label, 12)
+        data = pm.vault.get_encrypted_index() or b""
+        try:
+            asyncio.run(client.publish_delta(data, manifest_id))
+        except RuntimeError:
+            pm.sync_index_from_nostr()
+            pm.entry_manager.clear_cache()
+            pm.entry_manager.add_entry(label, 12)
+            data = pm.vault.get_encrypted_index() or b""
+            asyncio.run(client.publish_delta(data, manifest_id))
+
+    t1 = threading.Thread(target=publish, args=(pm_a, client_a, "from_a"))
+    t2 = threading.Thread(target=publish, args=(pm_b, client_b, "from_b"))
+    t1.start()
+    t2.start()
+    t1.join()
+    t2.join()
+
+    assert len(relay.deltas) >= 1
+
+    pm_b.sync_index_from_nostr()
+    pm_b.entry_manager.clear_cache()
+    labels = [e[1] for e in pm_b.entry_manager.list_entries()]
+    assert "from_a" in labels and "from_b" in labels