Pydantic operates on Python dictionaries, so questions about duplicate keys will come down to whatever the Python behaviour is. (I.e. if you have a dictionary l

Pydantic operates on Python dictionaries, so questions about duplicate keys will come down to whatever the Python behaviour is. (I.e. if you have a dictionary literal with duplicate keys, Python will select which key "wins" before Pydantic gets a chance to intervene.) When loading directly from JSON, it's possible to hook into Python's JSON-parsing mechanism and throw on duplicate keys, which probably ought to be the default given the number of security risks that come from this case.