@OddBloke did you find some time to look into this? Is this issue still relevant?

Looks like it is still relevant:

$ rustpython -c 'x=lambda a: None; x(a=1,a=2)'
[no output]
$ python -c 'x=lambda a: None; x(a=1,a=2)'
  File "<string>", line 1
SyntaxError: keyword argument repeated

I'd like to give a try on this issue.
I guess constructor for PyFuncArgs needs to check if the key already exists and raise SyntaxError

I think because it's a syntax error, you should do it in the compiler and check for the duplicates when you're compiling function arguments.