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,))
        else:
           d[k] = v
    return d

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

One alternative I wrote based on the solution posted by other users of this question is to convert those duplicates into an array:

def array_on_duplicate_keys(ordered_pairs):
    """Convert duplicate keys to arrays."""
    d = {}
    for k, v in ordered_pairs:
        if k in d:
            if type(d[k]) is list:
                d[k].append(v)
            else:
                d[k] = [d[k],v]
        else:
           d[k] = v
    return d

And then:

dict = json.loads('{"x": 1, "x": 2}', object_pairs_hook=array_on_duplicate_keys)

gives you the output:

{'x': [1, 2]}

Later one, one can check easily how meany duplicates an entry has by using:

if type(dict['x']) is list:
    print('Non-unique entry in dict at x, found', len(dict['x']),'repetitions.')