diff --git a/guarddog_nexus/constants.py b/guarddog_nexus/constants.py
index a66e4cf..24250c5 100644
--- a/guarddog_nexus/constants.py
+++ b/guarddog_nexus/constants.py
@@ -128,7 +128,7 @@ JSON_PATH_SEVERITY = "$.severity"
 # Webhook
 # ---------------------------------------------------------------------------
 
-RELEVANT_WEBHOOK_ACTIONS = {"CREATED", "UPDATED"}
+RELEVANT_WEBHOOK_ACTIONS = {"UPDATED"}
 
 WEBHOOK_IGNORE_NON_PACKAGE = "non_package_asset"
 WEBHOOK_IGNORE_NO_NAME_OR_VERSION = "no_name_or_version"
diff --git a/guarddog_nexus/harvester.py b/guarddog_nexus/harvester.py
index 2d6bf21..90e7df4 100644
--- a/guarddog_nexus/harvester.py
+++ b/guarddog_nexus/harvester.py
@@ -1,5 +1,6 @@
 """Harvester: download a package from Nexus, scan it, store results."""
 
+import asyncio
 import datetime
 import os
 import shutil
@@ -20,6 +21,10 @@ from guarddog_nexus.models import Finding, Scan, ScanStatus
 from guarddog_nexus.nexus_client import compute_sha256, download_asset, extract_pypi_info
 from guarddog_nexus.scanner import scan_package
 
+# Per-URL locks to avoid parallel scans of the same asset
+_url_locks: dict[str, asyncio.Lock] = {}
+_url_lock = asyncio.Lock()
+
 
 async def harvest(
     download_url: str,
@@ -42,16 +47,28 @@ async def harvest(
 
     package_name, package_version = info
 
-    active = await session.scalar(
-        select(Scan.id).where(
-            Scan.nexus_asset_url == download_url,
-            Scan.status.in_([ScanStatus.PENDING.value, ScanStatus.SCANNING.value]),
-        )
-    )
-    if active:
-        log.info("Already scanning this URL, skipping")
+    # Acquire per-URL lock to prevent parallel scans of the same asset
+    async with _url_lock:
+        if download_url not in _url_locks:
+            _url_locks[download_url] = asyncio.Lock()
+
+    lock = _url_locks[download_url]
+    if lock.locked():
+        log.info("URL already being processed, skipping: %s", download_url)
         return None
 
+    async with lock:
+        # Re-check DB in case another task already created and finished a scan
+        active = await session.scalar(
+            select(Scan.id).where(
+                Scan.nexus_asset_url == download_url,
+                Scan.status.in_([ScanStatus.PENDING.value, ScanStatus.SCANNING.value]),
+            )
+        )
+        if active:
+            log.info("Already scanning this URL, skipping")
+            return None
+
     scan = Scan(
         package_name=package_name,
         package_version=package_version,
diff --git a/tests/conftest.py b/tests/conftest.py
index 3fc582d..be65ca0 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -99,7 +99,7 @@ def sample_nexus_webhook():
         "timestamp": "2026-05-09T12:00:00.000+00:00",
         "nodeId": "test-node",
         "initiator": "admin",
-        "action": "CREATED",
+        "action": "UPDATED",
         "repositoryName": "pypi-proxy",
         "asset": {
             "id": "abc123",
@@ -117,7 +117,7 @@ def sample_nexus_component_webhook():
         "timestamp": "2026-05-09T12:00:00.000+00:00",
         "nodeId": "test-node",
         "initiator": "admin",
-        "action": "CREATED",
+        "action": "UPDATED",
         "repositoryName": "pypi-proxy",
         "component": {
             "id": "comp1",
diff --git a/tests/test_webhooks.py b/tests/test_webhooks.py
index b0aca44..b81848c 100644
--- a/tests/test_webhooks.py
+++ b/tests/test_webhooks.py
@@ -24,14 +24,11 @@ async def test_webhook_ignores_deleted_action(client, sample_nexus_webhook):
 
 
 @pytest.mark.asyncio
-async def test_webhook_accepts_asset_created(client, sample_nexus_webhook):
-    with patch("guarddog_nexus.webhooks._scan_in_background") as _mock:
-        resp = await client.post("/webhooks/nexus", json=sample_nexus_webhook)
-        assert resp.status_code == 200
-        data = resp.json()
-        assert data["status"] == "accepted"
-        assert data["action"] == "CREATED"
-        assert "/packages/requests/2.31.0/requests-2.31.0.tar.gz" in data["asset"]
+async def test_webhook_ignores_created_action(client, sample_nexus_webhook):
+    sample_nexus_webhook["action"] = "CREATED"
+    resp = await client.post("/webhooks/nexus", json=sample_nexus_webhook)
+    assert resp.status_code == 200
+    assert resp.json()["status"] == "ignored"
 
 
 @pytest.mark.asyncio