Duplicate keys

In more detail

Duplicate keys are the same name appearing twice in one object, as in {"a": 1, "a": 2}. RFC 8259 says the names within an object should be unique. It does not forbid repeats, but it warns that software receiving them behaves unpredictably.

In practice most parsers keep the last value without any warning. JavaScript's JSON.parse and Python's json module both return {"a": 2} for the example above. Some parsers keep the first value, and strict ones, such as an RFC 8785 canonicalizer, reject the document.

Because the first value is silently lost, duplicates usually point to a bug, such as a merge gone wrong or a hand-edited config file. Treat them as errors and remove them, even when your parser accepts them.

Example

import json

json.loads('{"a": 1, "a": 2}')   # {'a': 2}

# Reject duplicates with object_pairs_hook
def no_dupes(pairs):
    keys = [k for k, _ in pairs]
    if len(keys) != len(set(keys)):
        raise ValueError(f"duplicate keys: {keys}")
    return dict(pairs)

json.loads('{"a": 1, "a": 2}', object_pairs_hook=no_dupes)  # ValueError

Try it