I saw that pablogsal welcomed improvments to the parser's suggestions, so here are the messages for parameters and arguments lists I think should be written instead of the current generic "invalid syntax".

>>> def foo(*arg, *arg): pass
SyntaxError: * argument may appear only once

>>> def foo(**arg, **arg): pass
SyntaxError: ** argument may appear only once

>>> def foo(arg1, /, arg2, /, arg3): pass
SyntaxError: / may appear only once

>>> def foo(*args, /, arg): pass
SyntaxError: / must be ahead of *

>>> def foo(/, arg): pass
SyntaxError: at least one argument must precede /

>>> def foo(arg=): pass
SyntaxError: expected default value expression

>>> def foo(*args=None): pass
SyntaxError: * argument cannot have default value

>>> def foo(**kwargs=None): pass
SyntaxError: ** argument cannot have default value

>>> foo(*args=[0])
SyntaxError: cannot assign to iterable argument unpacking

>>> foo(**args={"a": None})
SyntaxError: cannot assign to keyword argument unpacking

>>> foo(arg=)
SyntaxError: expected argument value expression