chore(librarian): update configure_state_yaml.py to avoid duplicate entries (#14452)

This PR resolves the issue where duplicate values for `id` could be
added to `state.yaml`. When duplicate entries exist in `state.yaml`
librarian seems to provide an empty `generate-request.json` which leads
to the following error (see
googleapis/librarian#2186):
 
```
Traceback (most recent call last):
  File "/app/./cli.py", line 261, in handle_generate
    request_data = _read_json_file(f"{librarian}/{GENERATE_REQUEST_FILE}")
  File "/app/./cli.py", line 100, in _read_json_file
    return json.load(f)
  File "/usr/local/lib/python3.9/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/local/lib/python3.9/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/local/lib/python3.9/json/decoder.py", line 340, in decode
    raise JSONDecodeError("Extra data", s, end)
json.decoder.JSONDecodeError: Extra data: line 31 column 2 (char 755)
```

See output below for `google-cloud-dlp` and `google-cloud-eventarc`
which already exist in `.librarian/state.yaml`

```
partheniou@partheniou-vm-3:~/git/google-cloud-python/scripts/configure_state_yaml$ python3 configure_state_yaml.py 
Skipping package 'google-cloud-dlp' as it already exists in state.yaml.
Skipping package 'google-cloud-eventarc' as it already exists in state.yaml.
```

Author: parthea

scripts/configure_state_yaml/configure_state_yaml.py

@@ -48,7 +48,14 @@ def configure_state_yaml() -> None:
     with open(LIBRARIAN_YAML, "r") as state_yaml_file:
         state_dict = yaml.safe_load(state_yaml_file)
 
+    existing_library_ids = {library["id"] for library in state_dict.get("libraries", [])}
+
     for package_name in packages_to_onboard["packages_to_onboard"]:
+        # Check for duplication
+        if package_name in existing_library_ids:
+            print(f"Skipping package '{package_name}' as it already exists in state.yaml.")
+            continue
+
         package_path = Path(PACKAGES_DIR / package_name).resolve()
         api_paths = []
         for individual_metadata_file in package_path.rglob(f"**/{GAPIC_METADATA_JSON}"):