import json

your_data='{ \
  "group": { \
    "groupId": "1",\
    "groupName": "GTALUG"\
  },\
  "members": {\
    "1": {\
      "firstName": "Bob",\
      "lastName": "Jonkman"\
    },\
    "2": {\
      "firstName": "William",\
      "lastName": "Witteman"\
    },\
    "3": {\
      "firstName": "Lennart",\
      "lastName": "Sorensen"\
    },  \
    "4": {\
      "firstName": "Aruna",\
      "lastName": "Hewapathirane"\
    },\
    "4": {\
      "firstName": "William",\
      "lastName": "Park"\
    }\
  }\
}'

def on_duplicates(ordered_pairs):
    """Reject duplicate keys."""
    d = {}
    for key, value in ordered_pairs:
        if key in d:           
           print "CHECK FAILED - DUPLICATE KEY: %r VALUES: %r" % (key,value)                      
        else:
           d[key] = value           
                      
    return d

json_data=json.loads(your_data, object_pairs_hook=on_duplicates)