diff --git a/backend/open_data_connector.py b/backend/open_data_connector.py
index 1021d8a..32b9f64 100644
--- a/backend/open_data_connector.py
+++ b/backend/open_data_connector.py
@@ -160,10 +160,14 @@ def import_event_rows(rows: list[dict], source_name: str, notes: str) -> dict:
     imported_at = now_iso()
 
     db_rows = []
+    seen_ids = set()
     for item in rows:
         event_type = item.get("event_type")
         if event_type not in VALID_TYPES:
             continue
+        if item["id"] in seen_ids:
+            continue
+        seen_ids.add(item["id"])
 
         db_rows.append(
             (
diff --git a/docs/open_data_sources_nrw_live.json b/docs/open_data_sources_nrw_live.json
index d6ece13..541331d 100644
--- a/docs/open_data_sources_nrw_live.json
+++ b/docs/open_data_sources_nrw_live.json
@@ -45,7 +45,6 @@
       "source_name": "koeln_baustellen_notfall",
       "coord_crs": "EPSG:25832",
       "field_map": {
-        "external_id": "properties.Aktenzeichen",
         "lat": "geometry.coordinates.1",
         "lon": "geometry.coordinates.0",
         "event_type": "properties.Kategorie",