fix: reject duplicate keys in execution catalogs

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a0726e-5232-73f2-aaca-2c05ceb62efb

src/glas_harness/profiles.py

@@ -62,9 +62,28 @@ def _default_data_dir(kind: str) -> Path:
     return _source_root() / ("profiles" if kind == "profiles" else "registry/reins")
 
 
+class _UniqueKeyLoader(yaml.SafeLoader):
+    """Reject overwrites, including collisions introduced by YAML merges."""
+
+    def construct_mapping(self, node, deep=False):
+        mapping = super().construct_mapping(node, deep=deep)
+        seen = set()
+        for key_node, _ in node.value:
+            key = self.construct_object(key_node, deep=deep)
+            if key in seen:
+                mark = key_node.start_mark
+                # Key names and source snippets can themselves contain secrets.
+                raise yaml.YAMLError(
+                    f"duplicate YAML key at line {mark.line + 1}, "
+                    f"column {mark.column + 1}"
+                )
+            seen.add(key)
+        return mapping
+
+
 def _read_yaml(path: Path) -> dict[str, Any]:
     try:
-        data = yaml.safe_load(path.read_text())
+        data = yaml.load(path.read_text(), Loader=_UniqueKeyLoader)
     except (OSError, yaml.YAMLError) as exc:
         raise ProfileError(f"cannot read {path}: {exc}") from exc
     if not isinstance(data, dict):