I wrote:

I wonder, is this behaviour of {'a': 0, 'a': 1} documented (or tested) anywhere?

I've answered my own question here: [Python-ideas] dict literal allows duplicate keys https://mail.python.org/pipermail/python-ideas/2019-March/055717.html Finally, Christopher Barker wrote:

Yes, and had already been brought up in this thread ( I think by Guido). (Maybe not well documented, but certainly well understood and deliberate)

Thank you for this, Christopher. -- Jonathan