Report duplicate TypeVar definitions

[binitpoks-code]

Hi, thanks for assigning this issue to me.

This PR adds a focused binder-time check for duplicate legacy TypeVar definitions. Previously, Pyrefly accepted duplicate definitions like this without reporting an error:

from typing import TypeVar

T = TypeVar("T")
T = TypeVar("T")

After this change, the second definition reports a redefinition error.

The change is intentionally kept small and limited to the existing legacy TypeVar binding path. I also added a regression test in generic_legacy.rs to cover this case.

Fixes #3754

Test Plan

• cargo test test_tvar_duplicate_definition
• cargo test generic_legacy

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