make a dict/json from string with duplicate keys Python -

i have string parsed json or dict object. string variable looks :

    my_string_variable = "{                             "a":1,                             "b":{                                  "b1":1,                                  "b2":2                              },                               "b": {                                  "b1":3,                                  "b2":2,                                 "b4":8                              }                             }" 

when json.loads(my_string_variable), have dict second value of key "b" kept, normal because dict can't contain duplicate keys.

what best way have sort of defaultdict :

    result = {                'a':1,                'b': [{'b1':1,'b2':2}, { 'b1':3, 'b2':2,'b4':8 } ]              } 

i have looked similar questions deal dicts or lists input , create defaultdicts handle duplicate keys.

in case have string variable , want know if there simple way achieve :)

thank ^^

something following can done.

import json  def join_duplicate_keys(ordered_pairs):     d = {}     k, v in ordered_pairs:         if k in d:            if type(d[k]) == list:                d[k].append(v)            else:                newlist = []                newlist.append(d[k])                newlist.append(v)                d[k] = newlist         else:            d[k] = v     return d  raw_post_data = '{"a":1, "b":{"b1":1,"b2":2}, "b": { "b1":3, "b2":2,"b4":8} }' newdict = json.loads(raw_post_data, object_pairs_hook=join_duplicate_keys) print (newdict) 

please note above code depends on value type, if type(d[k]) == list. if original string gives list there error handling required make code robust.