[syntax-errors] Fix multiple assignment for class keyword argument

[ntBre]

Summary

Fixes #17181. The cases being tested with multiple keys being equal are actually a slightly different error, more like the error for MatchMapping than like the other multiple assignment errors:

>>> match x:
...     case Class(x=x, x=x): ...
...
  File "<python-input-249>", line 2
    case Class(x=x, x=x): ...
                      ^
SyntaxError: attribute name repeated in class pattern: x
>>> match x:
...     case {"x": 1, "x": 2}: ...
...
  File "<python-input-251>", line 2
    case {"x": 1, "x": 2}: ...
         ^^^^^^^^^^^^^^^^
SyntaxError: mapping pattern checks duplicate key ('x')
>>> match x:
...     case [x, x]: ...
...
  File "<python-input-252>", line 2
    case [x, x]: ...
             ^
SyntaxError: multiple assignments to name 'x' in pattern

This PR just stops the false positive reported in the issue, but I will quickly follow it up with a new rule (or possibly combined with the mapping rule) catching the repeated attributes separately.

Test Plan

New inline test_ok and updating the test_err cases to have duplicate values instead of keys.

[github-actions]

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[dhruvmanila]

Interesting, it seems that class patterns needs to be checked differently:

>>> class Foo: ...
... 
... match 1:
...     case Foo(x=1, y={"x": x}):
...         pass
...     case _:
...         pass
...         
>>> 

Here, the x in class keyword argument is not considered a duplicate from mapping pattern. This makes sense as the x in the class pattern is not binding but just trying to match against the class attribute.