import json

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)

json_data = json.load(open('william.json'), object_pairs_hook=on_duplicates)