diff --git a/README.md b/README.md
index 77f14da..2004c3d 100644
--- a/README.md
+++ b/README.md
@@ -84,10 +84,11 @@ LoD2 CityGML tiles can be converted to GLB per tile while preserving roof/wall s
    for f in work/cityjson/*.city.json; do
      base="$(basename "$f" .city.json)"
      input_json="$f/$base.json"  # citygml-tools writes a .json inside the .city.json folder
-     uv run cjio "$input_json" upgrade triangulate vertices_clean \
+     uv run cjio --ignore_duplicate_keys "$input_json" upgrade triangulate vertices_clean \
        save "work/cityjson_tri/${base}.tri.city.json"
    done
    ```
+   If cjio still complains about duplicate object IDs, inspect the source GML; `--ignore_duplicate_keys` tells cjio to keep the last occurrence.
 3. Split roof/wall semantics without reindexing vertices: `uv run python scripts/split_semantics.py`
 4. Export GLBs (tile-local coords; place with `export_unity/tile_index.csv`):
    ```bash