diff --git a/files/nats_registration_listener.py b/files/nats_registration_listener.py
index 14f121a..e03ad4f 100644
--- a/files/nats_registration_listener.py
+++ b/files/nats_registration_listener.py
@@ -316,6 +316,18 @@ async def main():
     DEDUPE_TTL = float(os.environ.get("DEDUP_TTL", "2.0"))
     recent_payloads: Dict[bytes, float] = {}
 
+    # ---- Per-device lock keyed by normalized MAC (prevents overlap for same device)
+    mac_locks: Dict[str, asyncio.Lock] = {}
+    mac_locks_guard = asyncio.Lock()
+
+    async def get_mac_lock(mac_key: str) -> asyncio.Lock:
+        async with mac_locks_guard:
+            lk = mac_locks.get(mac_key)
+            if lk is None:
+                lk = asyncio.Lock()
+                mac_locks[mac_key] = lk
+            return lk
+
     async def disconnected_cb():
         await log_status(f"[{ts()}] Disconnected from NATS.")
 
@@ -375,7 +387,9 @@ async def main():
         bell_prefix = ""    # ASCII BEL when action_next present (3x)
         netbox_time_ms = 0.0
 
-        if product == "fox100":
+        async def handle_fox100():
+            nonlocal host_suffix, bell_prefix, netbox_time_ms
+
             nb_start = time.perf_counter()
             try:
                 host, iface_id, dev_id, status_val, tag_slugs, action_next, action_last, action_next_timestamp, action_state = nb_lookup_device_by_mac(
@@ -597,6 +611,14 @@ async def main():
                 await nb_problem(log_status, f"[{ts()}] nb: unexpected error mac={mac!r} err={e!r}")
             netbox_time_ms = (time.perf_counter() - nb_start) * 1000
 
+        if product == "fox100":
+            mac_key = normalize_mac(mac)
+            if mac_key:
+                async with (await get_mac_lock(mac_key)):
+                    await handle_fox100()
+            else:
+                await handle_fox100()
+
         total_ms = (monotonic() - t_start) * 1000
 
         async with NB_PROBLEM_LOCK:
@@ -643,4 +665,4 @@ if __name__ == "__main__":
     try:
         asyncio.run(main())
     except KeyboardInterrupt:
-        pass
\ No newline at end of file
+        pass