json.loads allows duplicate keys in a dictionary, overwriting the first value

The rfc 4627 for application/json media type recommends unique keys but it doesn’t forbid them explicitly:

The names within an object SHOULD be unique.

From rfc 2119:

SHOULD This word, or the adjective “RECOMMENDED”, mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course.

import json

def dict_raise_on_duplicates(ordered_pairs):
    """Reject duplicate keys."""
    d = {}
    for k, v in ordered_pairs:
        if k in d:
           raise ValueError("duplicate key: %r" % (k,))
           d[k] = v
    return d

json.loads(raw_post_data, object_pairs_hook=dict_raise_on_duplicates)
# -> ValueError: duplicate key: u'1'

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