They get overwritten by the value of the dictionary that came last.

  >>> dict1 = {"key1": 1, "key2": "hello", "key3": [1, 2, 3]}
  >>> dict2 = {key1": 1, "key2": "world", "key3": [1, 4], "key4": "4"}

  >>> {**dict1, **dict2}
  {'key1': 1, 'key2': 'world', 'key3': [1, 4], 'key4': '4'}

I think it raises an exception.

https://www.python.org/dev/peps/pep-0448/

EDIT: yeah, nvm, it's what everyone else said.

Last pair wins, same as setting an existing key or doing an update() with an existing key.