How to ensure JSON has no duplicate map keys in Python 3.1+

json_no_dupes.py

from typing import List, Tuple, Any
import json

def ensure_no_duplicate_keys(object_pairs: List[Tuple[str, Any]]) -> dict:
	value = dict(object_pairs)
	if len(value) != len(object_pairs):
		raise ValueError("Duplicate JSON map keys")
	return value

if __name__ == "__main__":
	# The problem: this code "works", but silently ignores first entry
	print(json.loads('{"a": "123", "a": "xyz"}'))

	# This works entirely as expected, there are no duplicate keys here
	print(json.loads('{"a": "123", "b": "xyz"}', object_pairs_hook=ensure_no_duplicate_keys))

	# raises ValueError
	print(json.loads('{"a": "123", "a": "xyz"}', object_pairs_hook=ensure_no_duplicate_keys))