docs: concise explanation for check-yaml allow duplicate key feature.

README.md

@@ -87,9 +87,8 @@ Attempts to load all yaml files to verify syntax.
     portability to other yaml implementations.
     Implies `--allow-multiple-documents`.
   - `--allow_duplicate_keys` - Use this parameter to allow duplicate keys in yaml files.
-    In JSON mapping keys should be unique, in YAML they must be unique.
-    PyYAML never enforced this although the YAML 1.1 specification already required this.
-    Duplicate keys in mappings are no longer allowed by default.
+    Duplicate keys in yaml files are not allowed by default.
+    However, e.g. gitlab-ci.yml files may require this.
 
 #### `debug-statements`
 Check for debugger imports and py37+ `breakpoint()` calls in python source.

pre_commit_hooks/check_yaml.py

@@ -54,18 +54,19 @@ def main(argv: Sequence[str] | None = None) -> int:
     )
     parser.add_argument('filenames', nargs='*', help='Filenames to check.')
     parser.add_argument(
-        '--allow_duplicate_keys', '--allow-duplicate-keys', action='store_true', 
-        help=('In JSON mapping keys should be unique, in YAML they must be unique.'
-              'PyYAML never enforced this although the YAML 1.1 specification already required this.'
-              'Duplicate keys in mappings are no longer allowed by default.'
-              'Use this parameter to allow duplicate keys in yaml files.'
+        '--allow_duplicate_keys', '--allow-duplicate-keys',
+        action='store_true',
+        help=(
+            'Use this parameter to allow duplicate keys in yaml files.'
+            'Duplicate keys in yaml files are not allowed by default.'
+            'However, e.g. gitlab-ci.yml files may require this.'
         ),
     )
 
     args = parser.parse_args(argv)
 
     yaml.allow_duplicate_keys = args.allow_duplicate_keys
-        
+
     load_fn = LOAD_FNS[Key(multi=args.multi, unsafe=args.unsafe)]
 
     retval = 0

testing/resources/duplicate_key_yaml.notyaml

@@ -1,2 +1,2 @@
 hello: "world"
-hello: "planet"
+hello: "planet"

tests/check_yaml_test.py

@@ -27,6 +27,7 @@ def test_main_allow_multiple_documents(tmpdir):
     # should pass when we allow multiple documents
     assert not main(('--allow-multiple-documents', str(f)))
 
+
 def test_main_allow_duplicate_keys(tmpdir):
     f = get_resource_path('duplicate_key_yaml.notyaml')
     # should fail by default