Report duplicate keys in check_json

Raise ValueError and return 1 if json contains duplicate keys

pre_commit_hooks/check_json.py

@@ -1,7 +1,23 @@
 import argparse
 import json
+from typing import Any
+from typing import Dict
+from typing import List
 from typing import Optional
 from typing import Sequence
+from typing import Tuple
+
+
+def raise_duplicate_keys(
+        ordered_pairs: List[Tuple[str, Any]],
+) -> Dict[str, Any]:
+    d = {}
+    for key, val in ordered_pairs:
+        if key in d:
+            raise ValueError(f'Duplicate key: {key}')
+        else:
+            d[key] = val
+    return d
 
 
 def main(argv: Optional[Sequence[str]] = None) -> int:
@@ -13,7 +29,7 @@ def main(argv: Optional[Sequence[str]] = None) -> int:
     for filename in args.filenames:
         with open(filename, 'rb') as f:
             try:
-                json.load(f)
+                json.load(f, object_pairs_hook=raise_duplicate_keys)
             except ValueError as exc:
                 print(f'{filename}: Failed to json decode ({exc})')
                 retval = 1