Duplicate keys cause earlier TypedDict values to be skipped during type checking

Bug Report

When a dictionary literal used to construct a TypedDict contains duplicate
keys, mypy only type-checks the final value for that key. Earlier value
expressions are skipped entirely.

This can hide real errors because Python still evaluates every value expression
in the dictionary literal, including values that are later overwritten by a
duplicate key.

To Reproduce

from typing import TypedDict


class Payload(TypedDict):
    value: int


def parse(value: str) -> int:
    return len(value)


payload = Payload({"value": parse(123), "value": 0})

$ mypy --show-error-codes --no-incremental example.py
Success: no issues found in 1 source file

Running the program demonstrates that the skipped expression is evaluated:

TypeError: object of type 'int' has no len()

Expected Behavior

Mypy should type-check every value expression in the dictionary literal, even
if a later duplicate key determines the final value of the TypedDict field.
In this example, it should report an error similar to:

error: Argument 1 to "parse" has incompatible type "int"; expected "str"  [arg-type]

The same expression is correctly rejected when the target is a regular
dict[str, int]. The issue also occurs with a contextually typed literal:

payload: Payload = {"value": parse(123), "value": 0}

Actual Behavior

Mypy reports no issues because only the last value associated with the
duplicate key is checked.

Additional Context

The current implementation replaces the previously collected expression
without type-checking it and has a TODO to type-check all duplicate-key values:

https://github.com/python/mypy/blob/6eb141f03a8b43c274f15cc4978d617a71d8f37e/mypy/checkexpr.py#L839-L842

PR #15425 also mentioned repeated TypedDict keys as a known limitation, but I
could not find an existing issue that tracks it:

https://github.com/python/mypy/pull/15425

Environment Used to Reproduce

• Mypy versions used:
  • mypy 2.3.0 (compiled: yes)
  • mypy 2.4.0+dev.6eb141f03a8b43c274f15cc4978d617a71d8f37e (compiled: no)
• Mypy command-line flags: --show-error-codes --no-incremental
• Mypy configuration options: none
• Python version used: 3.12.8

mypy/checkexpr.py の839～842行目付近から始め、提供された mypy コマンドを使って example.py で問題を再現します。重複した TypedDict キーが以前の式を置き換える仕組みを追跡します。完了条件は、すべての値式が型チェックされ、parse(123) のケースで期待される arg-type エラーが報告されることです。