fix(map): reject duplicate YAML keys via a SafeLoader duplicate-check

tests/test_map.py

@@ -67,6 +67,27 @@ def test_minimal_map_without_optional_layers(tmp_path: Path) -> None:
     assert zone_cells(spec, "A") == []
 
 
+def test_rejects_duplicate_yaml_keys(tmp_path: Path) -> None:
+    """PyYAML is silently last-wins on duplicate mapping keys; a map that
+    declares `name` (or a legend entry) twice must fail, not keep one."""
+    body = _minimal_map() + 'name: "Test bis"\n'
+    with pytest.raises(MapValidationError, match="duplicate mapping key"):
+        load_map(_write(tmp_path, body))
+
+
+def test_rejects_duplicate_legend_keys(tmp_path: Path) -> None:
+    body = _minimal_map().replace(
+        '  ".": { type: floor, move_cost: 1 }\n',
+        '  ".": { type: floor, move_cost: 1 }\n  ".": { type: wall, move_cost: null }\n',
+    )
+    with pytest.raises(MapValidationError, match="duplicate mapping key"):
+        load_map(_write(tmp_path, body))
+
+
+def test_accepts_map_without_duplicates(tmp_path: Path) -> None:
+    assert load_map(_write(tmp_path, _minimal_map())).name == "Test"
+
+
 def test_rejects_ragged_rows(tmp_path: Path) -> None:
     body = _minimal_map().replace("  #.#\n", "  #.##\n")
     with pytest.raises(MapValidationError):